Engine or Tran Problem?
 

I have a 1965 mustang with a 289 V8 and automatic transmission. Recently the bellhousing on the transmission was cracked so we had it taken in to be replaced and ended up rebuilding the whole transmission. Now with everything back together, the car isn't running correctly. With the car in drive going down the road with at a constant speed the car will kind of lurch forward. I'm not touching the gas more than to keep a constant speed. It also will not start driving smoothly when I step on the gas, it lurches forward again. I have made sure the timing is correct and found one spark plug wire that was a little loose and fixed both of those but it is still not running right. I can't decide if the problem is in the engine or the transmission? Any ideas?