Sounds like the problem that i was having with my 87. It ened up being a clogged cat. Replaced everything like you did(except mine is carbureted) and finally figured it out when the clogged cat blew apart and removed the muffler off the back of it...lol.
Try unbolting the cat at the flange, and taking it for a ride. Its a little loud but it will tell you right away if that is your problem.