The only wiring you should need to swap is the engine wiring harness. Or do you plan to swap over the headlight motors, A/C, etc also?
Since I did a 4cyl-to-V6 swap in an '87 Fiero, I can tell you about a few tweaks that need to be done in the engine compartment:
-- On the '87 fiero, the heater return hose goes to the passenger side coolant pipe instead of the water pump. So you'll need to replace the fitting on top of the V6 water pump with a plug.
-- The brake booster vacuum hoses are different sizes. On my car, the V6 brake booster hose was bigger than the hose nipple on the firewall. I ended up using a short piece of the 4cyl vacuum hose along with an adapter to make it fit.
-- You'll need to chop about 6" off the end of the driver's side coolant pipe.
-- And of course, you'll need a V6 fuel pump.
Since you're working with an '87 4cyl, there are a few things you DON'T need to do, unless you really want to. You won't need to transfer the entire fuel line assembly over to the '87 Fiero. The only part you'll need to transfer is the hard lines attached to the V6 fuel rail. Everything from there down to the tank is compatible with the V6. You don't need to swap out the fuel level sender assembly, either. The V6 fuel pump will plug right in.
I guess that was a little more than you asked for.
