I've seen Sprint conversion package on eBay stateside for about $1,000 but then you have to do what ever to the head as these are used parts. Not seen for a while now. Remember Sprints where not imported into N/A as they would not meet emissions.
V8's from a Land Rover or more rare a Rover 3500 may be able to be picked up at auto wreckers for cheap but then (unlike UK people) you have to look for crossmember (rare and not cheap), a bellhousing (rare and not cheap) a PS rack (rare and not cheap), brake upgrade. Better to buy a TR8.
A Buick V6 is bolt in with John's cars kit (about $600 I think). You can buy a whole car for about $500 for odd fire or even fire with 5 speed tranny or automatic (plenty of ass kicking power for either). With the complete car you can even have fuel injection on some of the later ones. Uses same crossmember, brakes (although upgrade is preferable if you want to drive it hard.)
Since any of the three upgrades results in not an original car it doesn't make much difference which way you go unless you want the pseudo original look of the Sprint or TR8.
TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra
