If you are running stock internals you would want to stick with a single turbo setup. Twin turbos will generally run upwards of 20+lbs of boost.
You should be able to use either set of heads with a good kit.
You can rear mount it if you want a sleeper look or you can front mount it like most people do. The choice is entirely yours but the rear mount would require some extra tubing work with the exhaust.
Your numbers will depend on how much boost you run and what other supporting mods you add with it.
you are on the right track...if he is running stock internals and is planning on running them for a while, a single maybe 61mm is what i would go with. A single can run above 20psi as well as twins. And Twins can also be run on as little as 6-8psi of boost. I wouldn't recommend going with a rear mount...there is always a problem with spool up when going with a rear mount because there is just so much piping between the motor and turbo. And not to mention having to run a separate oil pump just to get the oil draining from the turbo back to the motor that is bound to go bad. The simpler the better.
My car is a 97 Cobra but has a GT short block with GT internals, and with a 61mm turbo the car makes boost before 3k rpm and makes 410rwhp on the dyno at 10psi. My b/f hasn't dyno'd his car, yet...but he has a 97 Cobra with stock heads, stock block, stock crank, stock style cams with manley rods and diamond pistons with twin 60-1 Master Power turbo's, and his car on about 7-8psi is about as fast as my car (I may pull him a tad). But since he's got the motor, he's gotten the car to see 21psi a few times, and trapped 151mph at the track...so it has a ton of versatility going on.
There is no way a turbo alone is going to get you 400rwhp. Iv'e got stage 2 cams, a .020+ bore, race heads, a positive displacement blower with 6psi and a custom dyno tune and I am only getting 375 at the rears.
No offense, but IMO, and it's been proven over and over again...turbo's are better and more efficient than blowers. His car on 10psi with say a 61 or even 67mm turbo will easily make over 400rwhp. Blowers need power to make power...the run off the crank of the motor. Turbo's work off exhaust heat and pressure. They are 2 completely different animals.
To the OP: If you are looking to keep it more of a street car making maybe 500-550rwhp max when all said and done, I would just stick with a single 61mm or 67mm. The car would run quite a while with stock internals and on 8psi with a good tune, and that would give you pretty close to 400rwhp if not 400. A single that size would be good for about 600-700 engine hp max, but will still be small enough to spool up pretty quickly.