I for one wouldn't run a turbo 4-banger with an auto trans in a RWD configuration... the drivetrain losses are too much for the tiny engine. A manual may be a better option.
I vote 3.9L swap, it's basically a 318 with two cylinders chopped off so it should hot-rod the same way. Parts will be hard to find but you can get creative reworking some small block stuff (headers, intakes, etc). I think with just a free-flowing dual exhaust it would nudge 200 HP, that's nothing to sneeze at in A-bodies with stock /6s pushing barely more than 140 in stock form.
