Well, start with a bellhousing from a 55-59 chevy or gmc 6 cylinder. Get the clutch pedal, clutch fork, clutch linkage, starter, starter pedal and linkage, flywheel from a 55-59 chevy 235... then decide on a transmission. There are plenty of transmissions that will work. Even Money says you will have to find a new driveshaft, or have one built. Don't forget you will need 2 new rear motor mounts. You might also need a new bellhousing crossmember to get everything else to bolt in.
For better gearing, you could swap out the gears in the rear to 3.38:1. You likely have 3.90:1 now, or you could swap the rear altogether. Of course if you find an OD transmission, you would not have to touch the rear.
The Hydramatic transmissions are highly sought after. If it were my truck I would makes sure the linkages are adjusted properly, and change the rear gears to something more highway friendly.
