The firewall and radiator core support are different between the '86 and the '88 bodies, the '86 won't take the long 4.0l without alot of body mods.

A good swap for the 2.8l is an EFI 3.4l V6 from a RWD Chevy. (Camaro) Pretty much a bolt in mechanically, and you could swap in the EFI system for alot more power. Howell may make a harness for the swap, I'm not sure.

You could strip a 3.4l motor down and install the 2.8l V6 manifolds, distributor and carb. If you go that way, you will need to install an electric fuel pump as the 3.4l does not have provisions on the block for a mechanical pump.