Cheers everyone, I'll pass on the Holden 5.0. The only reason I was considering it is that it will be quite a bit cheaper, but it's not worth it.
I'll just keep looking out for a newer MX-5 or Commodore SS. My first choice is the MX-5 because many more components can be used for the build (engine, gearbox, diff, driveshafts, wheels, gauges, steering components, spindles, rear suspension, mirrors, seats, brakes, etc.), but if a cheap SS comes up then I may just go that way.
Rob, I have talked to an engineer and he is highly dubious about shoehorning a V8 into a Locost. He says he's driven an alloy V6 engined one and the handling was very different (in a bad way), and that a V8 would be even worse. I'm not too fussed as this car will mostly be driven on the road, but I think I will go with a 4 cylinder just because thats what the car's designed for and what everybody recommends.
Here's a pic of the chassis I'll be fabricating, if anyone's interested, but it will be slightly modified for IRS:
