[Ant]

Whats wrong with a windsor alloy block with Canted valve heads, done properly, still 24" (600mm) wide, and 600+bhp available on pump gas, very light, and you can get some good Nascar bits for these engines, and pick and match to your requirements, but probably quite expensive as I have found out!!
I would have used an FE Alloy 427 in this car but the upper chassis tubes are to close to the engine, but I tell myself why settle for that old technology when I can have 2005 Nascar technology in a 1960's Block etc

Another good option that has become available, but still hard to get is the latest (wash my mouth out) GM LS7, 505bhp of one helluva engine, leave it stock and you have power and economy, I think one was up on ebay for USD$13000. My old Cobra had an LS7 BBC, and that thing would rip the t*ts off most other cars with Hilborne injection etc, and I would think the latest LS7 would make a good road race engine, just for 505bhp in a light package!