You really need to remove the box to do this properly as the bell housing should be taken off. You may be able to loosen the bolts with the box in place but then, you will have to deal with sealing the gearbox to the bell housing. You can remove the flange yokes from the gearbox and look at the area where the seal rides. If there is a groove worn into it, you can have the groove welded up and machined for a smooth surface. I’ve done this and it does work but best with new seals.
It’s really best to pull the engine and do it right. Take off the radiator and cross beam, disconnect the shift rods, throttle links, exhaust, etc. Since you’ve got the shafts out, it’s easy. A smooth floor (essential) an engine hoist and a chain around the water pump and you’re all set. Plus, if you pull off the bell housing you can clean out the old gunk that will be in the gearbox. Really, with a wide bodied car like that, it’s not difficult.