[joedoh]

i may have misread so bear with me but

a stock 53 3100 will be a torque tube rear

you dont have the stock rear (torque tube) any more. you do have stock replacement (drop) leaf springs.

I havent done much with torque tube axles but I know there is a special attachment from the axle to the leaf to allow the axle to move WRT the trans.

I seem to remember plates being sold/required to put an open drive axle on torque tube leafs. its possible you just need those plates.