You'll have to check this to be sure, but I know that on bikes from the 70s, a bolt-on axle was a smaller diameter than a QR axle. Therefore switching to QR if the bike originally came with bolt-ons only worked if you spread the dropouts a bit. If you can find a set of QR wheels to try, see if they fit inside the dropouts/fork tips easily. If not, then you should consult a bike shop to see if the frame can be modified to fit.

If you're having trouble loosening the bolts to get your wheel off for a tire change, look for a longer wrench.