Ok fellas, buddy of mine has an 81 square body with a 350, cam isn't stock but not radical. The previous transmission was burnt up by a previous owner. We swapped another th350 (stock convertor) into the truck from an 85 square body he has. The trans worked fine before the swap. He changed the filter and refilled with fluid. Vac line is hooked up to full manifold on the carb. Fluid is full. Now for the issue. When he puts the truck in gear it pulls load off the engine as it should, but he has to very lightly feather the gas to get the truck to barely move. He says when he gives it normal throttle it stops moving and revs as if in neutral. He's rechecked fluid multiple times and its full. I know near to nothing about automatic trans so any help would be much appreciated.