A couple of questions, did the reverse button work for the two months since you got it back and quit suddenly or was this the first time you had the car out? If it had been working and quit, probably the problem is not a cable adjustment, because once they are properly adjusted, they usually stay that way unless the cable breaks. As the car will still go into drive it is unlikely that the cable is broken. If this was the first trip out where you used the reverse, then the cable was not set properly by the shop that did the work.
I would think that a reputable shop would adjust the cable for you as this would be a no cost warranty repair, so take it back and explain the problem to them. The other problem could be that some small linkage part in the push button control has come lose or broken, in that case the push button unit may have to be disassembled and repaired, you would probably wind up paying for that type of repair unless the restoration shop also rebuilt the push button unit. These push button control units were normally very reliable and seldom broke, but it can happen after all these cars are now 55+ years old.
The good news is that a faulty cable or cable adjustment or bad push button unit will not usually damage the transmission, only disable one or more of the selector settings.
As noted above, the FSM gives detailed instructions on how to adjust the cable, but given the recent repairs, I would take it to the shop that did the repairs first and report back.
Dave
