The problems are these: first, the intake manifold from the 3.4 wont fit. It will either hit the thermostat housing if you put it on the way its supposed to go, or it will hit the distributor if you turn it around like on a Fiero. Second, you could use the 3.4s DIS ignition system, but it requires that you use all the wiring from the 3.4 and its computer and you wire those into your car. If you use the 2.8 ignition system, you simply put that system onto the 3.4 and use the 2.8 wiring harness and it all drops back into your car just like a 2.8 would.
D.I.S. has already been talked about on this forum. Just do a search. I have a 3.4 and with the Camaro intake flipped around. I also have the D.I.S. system. You need to modify the Fiero's harness a little bit, for the ignition system. On the computer side, repin the connectors at the computer to run a 1227730 ECM. One other way, that might work, is to change the timing codes in the Fiero's computer chip, From 10* BTDC to 70*
As far as stock manifolds go, they all flow about the same, and the Fiero manifold is the only one that fits turned around. The rest would have to be modified. There's a thread by Triker in this section about improving the stock manifold by cutting into it and making some modifications.
