trans problems

ok my buddy john has a 94 z28 and it trans burnt up... we went to the junk yard and got trans out of a 93 trans am i believe... now as ive heard 93s arnt electronicaly controlled but im positive it said it was built in 94, now it could be a 94... but heres the problem... The trans shifts from 1st to 2nd fine. But then when it hits 3rd and hits 2 k rpms it seems like it slips and revs to 4 grand and grabs and stays at 4 grand wont shift to overdrive. Goes higher than usual.. i remember on the center console of the trans am there was a shift perform button of some sort.... but all the other plugs and everything match up fine and im pretty sure he put the proper amount of fluid in it

Maybe the tranny its self isn't in the greatest shape cause it sounds like its slipping.