If you did a load test on the battery (not just voltage test) and it checked good, try replacing both battery cables and clean the ground from battery to block. The cables can develop internal resistance due to heat damage (like trouble shooting and running the starter too long).
It doesn't take much to spin these 6 bangers, but a large voltage drop will cause the starter to drop the bendix back to rest position.
Carter BBD feeding 4.2 Nuttered 6 banger = 22 MPG Hwy
If it was easy, everyone would be doing it.