Yeah yeah yeah I know I should finish my Spit6 before starting on another big project, but it's just such a damn cool idea. Plus, I don't think anyone's done it before (at least no-one who works the internet).
I've done the research and it should bolt up to a Triumph TR7 LT77 5-speed, leading to a Subaru R180 diff from a late-2000 Subaru Impreza STi (if I get the right year it should come with a clutch-type LSD). Given the 3.545 ratio that should give me a theoretical top speed of 144mph (which given roughly 230bhp and such a dinky car should be easily reached). If I want a higher v-max then I can use the crown wheel and pinion from the front diff of a Nissan XTerra (not sure they sold these in the UK) which would give a ratio of 3.13 and a theoretical top speed of 163mph.
I've got the option of the earlier slanted B202 engine from a 900 turbo which is slanted so probably easier to fit under the bonnet, or the more easily tunable B204 and B234 from the Saab 9000 which can easily be taken up to 270/300bhp respectively.
I'd probably go for the B202, purely because it seems to fit the car better aesthetically (and the problems it has in fitting in the Triumph chassis seem easier to solve than the later engines). Even with the lower power figure, it should still have a better power-to-weight ratio than a a Mk Viper, or a C63 AMG, or a 991 Carrera :)