I have a muncie m-21 in my 69z that has a problem with 3rd gear clashing only in the heat of the summer and mainly on downshifts it shifts fine in the fall and stays in gear. I'm considering having it rebuilt or rebuilding it myself? Any input? I was also considering swapping to m-20 for the better fisrt gear, but am worried about tire spin at the track? Lastly has anyone dealt with JT-muncies out of mass?<He seems to be reputable in my area just looking for a little more input thanks, and sorry this is so long.