700r4 shifting problems?

I had a so called tranny shop putt in the 700r4 and a 350 engn in my 68 c10. the truck has no revers, it shifts just fine for the first mile then it goes nowere.. when you let off the gas it seams to catch a gear and drive a little, but then slipps again. It had reverse whn it left the shop This guy has had my truck for three mnths. One excuse after anther with this guy during this tranny engine swapp. I really dont want to take it back there....