loss of power need help 89 lx 4cly

ok so the trany wint bad it was an overdrive 3 wier and i replased it with a 88 3wier no over drive and had amco do it and now the stang has like a 50%loss of power and amco ses the trany is fine i also replased the timing belt i think it is intime and so duse my dad but some thing is not right it take like hafe a miel to git up to 60mph and i dont thik that is right also if you trn the car on and let it idel it will stal out in like 2min any thouts pleas help lusing my mind