Wiring the 4.9
For schematics and other info about making the wiring harness I'll direct you here to my general wiring info page. The 4.9 wiring is handled the same way as any other conversion, but there are a few things that should be known before getting started, so here they are. The following information pertains to the stock Cadillac PFI ECM
All 4.9 liter Cadillacs were equipped with the Passkey theft deterrent system. You'll need to either use a GM Passkey module, build or buy a Passkey bypass, or get the chip reprogrammed to turn it off.
There are some wiring differences between the Deville/Fleetwood and the Seville/Eldorado. Just be aware of them before you get started. I prefer to use the Deville schematics for several reasons, most important being it has a single O2 sensor, and it has simpler Serial data connections.
The power steering pressure switch input to the ECM can not be ignored. If it is left open, it will disable fuel trim. It needs to be connected to a working psps, or to +12v in the case of the Fiero.
For scantool data transfer, you need to connect ECM terminal A8 to terminal M of either the Fiero or Cadillac ALDL connector, and scan as a Deville (VIN fourth digit C).
You will likely get several error codes using a stock Cadillac chip. Some of them can be eliminated using creative wiring techniques, others will require a custom chip. None that I have seen have caused drivability problems, but I like to correct them anyway.