Need Trans Help!

I have a 95 Mustang GT that I got a few months ago and I've had problems with it since day 1. The night after I got it, it was stuck in 2nd and my freind had to kick it out of gear. After that I noticed that it started to act up and it wouldnt go into 2nd and eventually 3rd. Most of the gears are either hard to get into or wont get into at all. The ones that go in easier than others are 1st, 4th, and 5th. My dad parked it and left it in 3rd and it got stuck in gear again and we had to get it out with force again.The car was modded by the previous owner and I dont have contact with him to get a list of what he did. The only things I know that were done was full exhaust, headers back, it was cammed, and it has a tranny conversion from auto to 5spd.

What I am asking is
a) do you know what may be wrong with the tranny?
b) if it is shot, which I think it is, should I put a performance trans or should I put a stock trans from another mustang or get it rebuilt?
c)how much do you think it would cost? The car was taken off the road and I cant take it to a shop and I dont wanna pay a towing bill to tow it to a shop and have them charge me to look at it

Any suggestions at all will help and I appreciate all help, Thanks

-Dante

it has a lot of free play and its easy to start and shift, its getting it in gear thats the problem

I'd say about an 1", also the pedal is hard to put to the floor compared to other standard cars i've driven