904 won't go into passing gear...

When the throttle is wide open, the throttle pressure arm that is on the shaft that comes out of the transmission, should be at the limit of its travel, too. They shouuld "bottom out" simultaneously, as you open your throttle all the way.

I'd bet that either your throttle linkage is not pulling your throttle blades wide open, or if it IS, then the linkage between the tranny and the carb needs adjusting (shortening.)

Once you've made sure that the throttle cable is opening the carb all the way, THEN check and see if the throttle pressure arm on the transmission is being moved to ITS limit, too. If it's not, simply adjust the threaded portion of its linkage so that it is.

Maybe Ma Mopar had rocket scientists writing instructions for this operation, but for the rest of us, this will work. Insuffucient throttle pressure WILL cause clutch pack slippage, but with preper throttle pressure linkage adjustment, the slippage may stop.

I realize that most, or all of what I told you is just re-capping what others already said, but it never hurts to hear it again.
Good luck...
Let us know how this ends up. please.

Bill, in Conway, Arkansas