swap ? 4cyl to gt-40p wiring issues

[jas7865]

I did a 4cyl to gt-40 swap with the p heads no mods on engine except exhaust , air box and gt 40 intake ,used wiring harness out of 93gt, computer im unsure what its off of but i bought all of the swap stuff from midwest mustangs a very reputable business. Well heres my problem when I start the car it surges way up to 2000 rpm and never really comes down to where it should, and the factory tac and speedometer are way off ,switched gear end of speedometer but the results where the same could all of these electrical problems be effecting the way this thing is running,also I have removed the air pump because the original 5.0 out of my 88 lx had emission tube running to the heads and exhaust and the gt 40 didnt ,I used the 88 5.0 for a doner car. any help with my electrical and engine problems would be greatly appreciated.