Got an oil cooler? Fit one. It's surprising how much difference they can make. I bought one about a foot square at Sodbury for £3.
As has been said, a rear mounted rad should cool fine. I'd trawl the scrapyard for the biggest fan(s) you can make fit, and make sure they have a good seal on the rad. A big ali rad from a modern van, SUV, 4x4 or people carrier could be a good stop before calling Summit, although their ali rads are ludicrously cheap.
You say the engine runs sweet - is it running lean? Slightly lean will give good response & power but RV8's don't like being lean, if there's any way you can richen it up, give that a try. Replacing the ECU coolant sensor with a resistor to make it think it's a bit cooler than normal can richen the mixture a bit - 400 Ohms would make it think it's about 70C, 500 Ohms is about 62C, 700 is about 50C, 1k is about 40C. You could get sexercisey and use a 4K pot to get the full range of twiddle, with a switch to reconnect the original sensor when the novelty wears off.
Oh and don't listen to the diesel boys (if you can hear them over the rattling), you'd only spend all your time changing cambelts instead