The firewall mod it trivial, the real issue lies with the hood and radiator support. The 4.0L requires a hood with the latches on both front corners, not one in the middle. I know the 85 is center latch only, I haven't ever had a look at an 86.
I did the 4.7L stroker (4.0L size) into an 85. I had to cut the headlight buckets and brackets from a donor vehicle and move them over along with a new hood. It took a bit of sheetmetal work, but not bad. I hear a lot of good things from the guys that do the 3.4L swap, they say it is simple and easy. The 2.8L is not known for having enough power or for lasting very long.
Most people who work on enhancing the 2.8L don't end up happy. For what a new Webber would cost, you can most likely snag a 3.4L from a salvage yard and end up way ahead.
The is practically no limit to what CAN be done, just a limit on your pocket book. An LS1 would be a nice upgrade. :jester:
Good Luck