1971 Chevy El Camino Transmission troubles

After taking the truck off the road for transmission and differential leaks, also I had failing brakes, I have repaired the leaks and brake issues (with everything still attached to the vehicle), but now the truck won’t move on its own. I have a stock 350 small block, and the turbo 350 trans.

The symptoms are:

1. I stuck it in reverse and moves with extra acceleration, but not that well.
2. You put the truck in drive, and at first it moved, but not so great, then you will stop, put it in Park, and go to put it in drive, and you can tell the transmission does not engage, and will not move, no matter how much revs you give it, and if you drive it in reverse for a little, and stick it in Drive, it will start to move again, and you can tell when you give it some gas, it does not sound like the transmission is not coping well at all.

I can perform a shifter adjustment myself, which i have not tried yet, but if you think you know exactly what the problem is, tell me before I break something.