I had the exact same problem with my 73 as soon as I dumped a ton of money into it! The trans needed rebuilding but I found a decent 904 with a shift kit at a swap meet for $200. I threw that in the car and was going to rebuild the original but the brand spanking 'used' one did the job fine. I had some similar work done to my 318. I had a high rise dual plane manifold with a 650, electronic ignition and the purple shaft .486 cam. I also added a set of 360 'J' heads that I had cut down .040 for upping the compression and the thin gaskets and a set of Dynatech headers. That engine purred like a kitten and certainly had a lot of power out of it. I took it to the track and the best ET was 15.0 @ 102mph. All that engine work is great but the stock drivetrain kills it. I installed a 4.10 posi but the car was stolen before I had a chance to run it again.