When I shift my car into reverse, I feel out trying to go backwards but it didn't move. why its that?

I recently put a new motor into my deville. Turns out, the motor was out of a Seville. No big deal because its still a 32 valve 4.6 North star right? I put new coil packs in it and changed all of my sensors. I put a quart of trany fluid in it after realizing it was low and it still won't reverse. It feels like its not getting enough "umph," as my dad would say. I throw it in drive and it goes forward but acts as if it isn't getting enough "torque" .... It accelerates slowly. I gave up messing with it because the engine seems to be working perfectly. I don't know what to do. Should I drop the pan and change my transmission filter? I have the same transmission in it that I had before the last engine blew a head gasket. Both the newer engine & transmission were sitting in The back yard for about 8 months before I took out from the prick I paid to fix it and just did it myself.