My bet is your bellvue spring broke in the rear drum and the forward gear clutches are mechanically applied with no return pressure from the bellvue spring. This is why the car wants to move forward in every gear, including park and neutral. In reverse, the low/reverse servo is applied, coupled with the front clutch. So when the front clutch, rear clutch, and low/reverse servo are all applied at the same time, you have a trans brake application. Hence why in reverse it won’t move. The transmission will need removed and disassembled to fix that issue. I would suggest a billet aluminum forward drum piston (.875” tall if stock drum and hub), metal spacer ring, and late model bellvue spring. That’s the fix. It’s also possible the bellvue spring didn’t break and the piston had so much play between the piston and bellvue spring, that once the piston applied, it could have popped out of the bore. Either way, the taller billet piston, metal spacer ring, and late model bellvue spring are the fix. Looking at $150ish total for those few components.
