tallpaul:
Ian is right when he mentions that this car does not want to be swapped. Forget V8..this car was pretty much designed around one drivetrain.
What I am doing (drivetrain and suspension wise) is designing a whole new car that will have a Volvo P1800 shell along for the ride. The finished result will have more Monster Miata DNA than Volvo when it is done.
The issue with swapping a V anything into a P1800 is the front suspension: The upper a-arms in particular are a problem since they intrude into the engine compartment and leave no room for headers at the front cylinders.
The front crossmember is also massive, forcing the engine upwards and not tolerating front sump engines.
You also do not have a whole lot of headroom in that engine compartment, so while a V6 like the S10 3.4 would work, anything with OHC might be a bad idea if you want it under the hood.
The classic P1800 swap involves shoving the engine WAY back into the firewall. This involves a lot of work on things such as wipers and HVAC systems. Also, the foot-room gets a bit compromised; not for length, but for width.
The Ford AOD gearbox is a worse-case scenario for a swap. A T5 would pretty much leave your footspace unaffected. If the car were for me rather than the wife I would have used a T5 with the S10 short tailshaft and maybe tried to see if I could keep the two part driveshaft.
Keeping a live axle would also make life easier.
As a rule, swaps do not make perfect cars; we just ain't that good at this stuff compared to the teams of engineers that built the things in the first place. The other thing is that we lose patience when the development pha$e starts (car is on road and feels/handles like a turd).
Believe it or not, the Miata envelope is not too far from the P1800 in terms of weight and wheelbase. By using a suspension that was designed by Mazda get brakes and geometry that should be close. By following a V8 conversion recipe that has been developed by the Miata crowd, I take advantage of all their development.
This is all just fun anyways, so go for what tickles your fancy and enjoy the process.