First you need to verify what transmission and tcase you have. 78 didn't come factory with a 304/4 speed combo. Something has been swapped along the way.
Could be an original 258/t18/d20 combo in which case I would fix the transmission. Or you could sell it for more than you'll spend on a D300. Someone could have just as easily swapped in a T176/d300 in which case you already have the tcase you're looking for.
Lots of other possibilities, just throwing out a few for demonstration. Before buying any more parts, make sure you know what you have.
77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal