droptopred

I just got a 94 5.0, 5 sppedwith about 100k miles on it and minor suspension mods, and i have some questions.
It feels like there is something wrong with the tranny, I hear grinding under the car sometimes, and also when i go about 50 in 5th gear the shifter shakes pretty bad.
Another thing that bugs me is when i let go off thee gas the car stays up on the rpm's for about 15 seconds or so before it drops, and also the ac only works on full blast.

Any suggestions as to what i can do, i have a 95 gt that i wrecked so i plan on using parts from there.
Iam looking to make this a track car but also drive it around the city once in a while.

All suggestions will be great.