If you are going to move the battery up front, take the Riviera's battery cable ( from back seat). ON either the Riv or the Ultra, take the extra Relays that are attached to the firewall. Always come in handy. The Riviera has a unique engine harness arrangement, where it plugs into a connector on the firewall, which then connects to the ECM and the rest of the wiring. A little more complicated than say a Grand Prix harness, but still doable. When I did the Riviera, I wired the connectors to end where the Fiero Battery now sits. Makes it easy to take the engine out, but like I said the wiring to get there is more complicated. I took a lot of the rest of the wiring from the Riviera just to have wire to extend where necessary. The Riviera also has lots of parts for "part out" that might recover some of your money, if they are in good condition.
I found the Grand Prix harness the easiest to work with- It's nearly a direct plug in- just a few wires to connect to the Fiero ( I used the underhood Fuse box from the Grand Prix too- just took it all out as one assembly). The GP also has the correct front accessory drive, as well as the right Oil Filter adapter.
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'87 GT , '00 3800 Series II SC, 4t65e, Vue Power Steering. (SOLD)