I too would suggest 45 ft lb. This is factory spec at least for '70 with similar U bolts. Too tight will bind it up and make it "pop" on bumps.
Always a good idea to re-check torque after driving a while.
Where does the pop come from? Do the leaves move relative to each other within the few inch span between the u-bolts?
I do believe there is some movement of the leafs relative to each other. If it is too tight, pressure builds up then it moves/pops. I over torqued my U bolts once and had the pop on bumps. Back them off and correct torque fixed it.