Cooling mightn't be directly related to making horsepower, but then again all the power in the world is of no use if your engine is boiling and puking up its coolant. The amount of heat that the cooling system has to handle is directly proportional to the power produced, so if you've doubled your engines output you've also doubled the amount of heat you need to dump. Similarly, it's unrealistic to expect a radiator originally intended to cool an 80hp grey to adequately manage a 250hp screamer. Having said that though, provided you can arrange decent airflow the radiator doesn't have to be massive. Logically you would use a radiator originally designed to cool an engine of similar power to your modified six eg. a stock 308 unit. When selecting your radiator it's generally more important to get something with a bigger frontal area rather than a very thick unit with lots of rows of tubes. Two or three rows should be enough - cores that are too thick or with densely packed fins restrict the airflow too much and are actually counterproductive. You'll probably have a fair bit of sheetmetal work to do to the car ensure that the entire area of the core is open to air flow.

Any car that will be used at low speeds or in traffic will need a fan. It's often difficult to get a fan working effectively, and on many cars the fan ends up being too close to the radiator. With early model Holdens in particular there's not much room at all. Try this fan flow test: with the engine running at a fast idle take a portable air flow velocity sensor (a.k.a. a piece of wool tied to a stick) and check the flow at different positions over the core. You'll probably find that there is a fairly big spot right in the middle where there is no flow, and also little or no flow at the corners. Effectively there's only a ring shaped area where there is any real airflow. Do the sums and you might well find that over 30 or 40 percent of the radiator is doing nothing to cool the engine.

Ideally, the fan should sit 150 to 200mm back from the radiator, and in a close fitting shroud, though few cars have this much room available. So long as the fan is far enough back the shroud will direct the flow over the entire core. Of course, this is of no use if the radiator is sitting an inch from the water pump shaft, and in these situations you'll probably be stuck with an electric fan. I've had good results from the old style stainless flex fans mounted as close as a couple of inches from the core, and closely shrouded.

Electric fans are quite popular, and they do have the advantage of only running when required. The other advantage is that they free up quite a few horsepower normally used to spin a belt driven fan. Unfortunately they are usually mounted too close to the radiator to cool much of the core - the aftermarket ones are particularly bad in this regard. Aftermarket fans often use some sort of cable-tie-through-the-radiator-core method of attachment that looks like trouble waiting to happen. If you have to run an electric fan check the wreckers for OEM setups that are generally much better designed, being spaced further from the core and incorporating some sort of shrouding.

Water pumps can also be a bit problematic, specifically some of the aftermarket ones with the open impellor. The best plan is to use an original Holden pump with the cast iron impellor, or at least use the closed C.I. impellor on whatever pump body you have. It's important to make sure the clearance between the impellor vanes and the pump body is quite close, no more than say 10 thou. Too much clearance here allows the water to short-circuit from the high pressure side back under the impellor to the low pressure side. Machine the face of the pump body if necessary to make it flat so the clearance is uniform right across the face of the vanes. There is a variety of shaft lengths and flange sizes available; something to be aware of when choosing a replacement.

It's important to run a thermostat to control engine temperature - a too-cold engine won't last very long, and it won't make as much power. When everything is working properly there will be a temperature drop of around 5 to 10 degrees C across the radiator, and the pump inlet temperature will be less than the thermostat setpoint. Overheating coupled with insufficient temp drop indicates lack of airflow or radiator area, while overheating with too big a temp drop indicates a lack of coolant flow. Readers of hot-rodding magazines have probably come across the theory that excessive coolant flow leads to overheating, and as usual with these magazines this is pure drivel. The theory states that the fast flowing coolant doesn't have time to pick up or dump heat but conveniently ignores the fact that the coolant will be making more laps of the system in a given time. This exactly cancels out the shorter time to transfer the heat, and in fact the greater turbulence in a fast flowing system will aid heat transfer significantly. Of course the higher flow takes more horsepower to pump so it's a trade-off. Turning the pump too quickly can sometime lead to pump cavitation and a net reduction in flow, and perhaps this is where the old wives tale started.

Not all thermostats have a small bleed hole and jiggle pin, and this has caused some people a lot of grief on initially firing up a freshly built engine. Without the bleed hole there may be no other way for air to escape from the top of the block and the head so these areas may remain dry despite the radiator being completely filled. If your thermostat doesn't have a hole then it may pay to drill one or at least make sure you have some way of bleeding the air from the head side of the thermostat.

Thermostat showing bleed hole
Thermostat showing the bleed hole with jiggle pin. This little hole can be critical to the engine surviving the initial startup.

You'll probably want to run some sort of anti-corrosion/anti-freeze additive, just be aware that plain old water has the highest heat transfer capacity so don't mix the coolant in higher concentrations than necessary, and definitely don't use the old style soluble oils that can inhibit heat transfer.

One more thing - in standard form a vee-belt is used to drive the fan/water pump/alternator. This generally works very well, being quiet, compact and reliable, even with poorly aligned pulleys. Replacement belts are available everywhere. But for some unfathomable reason quite a few people instead use a "Gilmer" belt setup having none of the above attributes. Go figure. Anyhow, don't do it, it's dumb.