Your sig line says it all: 1968. That is the last year they used those weeny little 8mm bolts to hold the driveshafts on. I had nothing but problems with these in my 40, and ended up swapping in flanges and shafts from a 69. No more problems.
Chances are, this has happened more than once to the PO of your rig, and the flange holes are also rounded out, making it even harder to keep the bolt tight. If this is the case, consider drilling out the flanges to to the next larger SAE bolt size, finding a bolt with an appropriately long shoulder to fill the holes completely, and mount them.
