It's great to hear about such a project! First of all, you might visit the turbo124 forum, several guys there have ~400 Hp FIat 124 Coupes. I believe most of them use the later Fiat 16 valve head and a turbo. So the block should be able to take the power. You would need forged rods and pistons, doweled flywheel with upgraded flywheel bolts (we carry them).
For transmission consider the Mazda 6 speed, or I think Borg-warner makes a 6 speed, maybe the T-56? You'd definitely need a limited slip.
You are starting off with the 'wrong' body, as the Pinin is the heaviest of the Spider bodies. Most of the extra weight is removable, but some of it is not. I doubt you could get a street Spider much below 2000 lbs. It all depends on how much discomfort you are willing to put up with
[Lack of insulation. top, etc] You should also consider body stiffening so the suspension works better. I have some ideas, send me a PM if you'd like to discuss.bye,
The square exhaust and roll bar,wood dash...ick!!I guarantee you with that lump in the front and the jacked up suspension,a Exige would blow that thing off a road course.Might beat the Lotus in a 1/4 mile though.
Good luck regardless which ever road you decide to go down and keep us updated!