700r4 Issues

[N9nefingers]

The backstory...bought a 1976 K5 Chalet and showed up pretty much a wreck. Got the rust taken care of and started on the drivetrain. Built a 350 SBC with an RV Cam, nothing fancy just a nice, new SBC. I also picked up a 700r4 and a NP241. Got those off CL, guy said they worked, rolled the dice. So I got it all in the truck and hooked up and took it for a drive. If I am on the gas I get a hard shift at around 20-23 MPH and the truck keeps pulling. Only driving around my neighborhood for now so 2nd is all I've gotten to so far. The issue I'm having is if I get off the throttle before the shift the truck seems like it has no gears...RPM goes up but the truck doesn't really move any faster. If I come to a stop and take off again it pulls hard and I get a shift and keep pulling. Any ideas? Transmissions are not my strongest area. Thanks in advance.