tranny swap

My 97 dodge ram 1500 is only getting to 1500 rpms and stops even with the gas punched what could my problem be? I did a tranny swap and then a transmission place replaced a o2 sensor and camshaft sensor and wires to my speed gauge i think and a few other things. Ran good for for months then i got my glass pack a taken out and put straight pipe where the glass packs were and 2 weeks later it stopped working and now is doing this thing where it only get to 1500 rpms