904 help, won't move

My son and I are putting together an 84 D150, 5.9 magnum, original lockup 904 with shift kit. Drove great 1st few test runs. Now it engages drive and reverse but won't move unless revved up. Put in a new torque converter, no debris in pan. It blew the pressure fitting at the radiator, pump is good sprayed all over. Tested, zero pressure drop through radiator and back to transmission. Reads high on the dipstick even though i know it is a quart low.. something plugged in the valve body?