[84LX89GT]

If you'd use the 4 banger as a sleeper building the engine with either the forged internals or 2.5L bottom end and nitrous would work pretty well and you wouldn't have to do as much rewiring, plumbing and parts finding as you would if you bought a turbo coupe engine. If you drill into the lower intake and use direct port "wet" nitrous kit you can get some serious power out of that 4 banger and not worry about distribution among cylinders....but that would be more expensive than a normal single nozzle in the intake tube or into the upper intake manifold. I honestly would go steeper than 3.43 ring and pinion, 4.10 would help you make up the torque loss of the engine and keep it up in the power band better.
With the trans, i don't believe you'll really need to rebuild it since the 4 banger T-5 has a steeper 1st gear to help get off the line, having the V8 ratios will increase the torque rating of the trans but will reduce torque mulitplication for low engine speeds. Also you won't max out the torque rating on launch with a 4 cylinder because stock they make like 135 lb/ft and you may get it up to 200 or so lb/ft with alot of work, and i believe the 4 banger T-5's are rated at 260 lb/ft so you aren't very likely to scatter the trans.

Just some ideas