The general guideline is to size the fusible link 2 wire sizes less than the main wire... 12 guage wire would use a 16 guage fusible link for example.

One sizing consideration is alternator output... a '71 would likley have had a 50 or max 60 amp alternator and a 12 guage charge wire. The 12 guage wire + 16 guage fusible link could be considered OK for intermittant current up to 50 or 60 amps so that would be the fuse size. Call it 60 amps. Continuous capability is more like 30 amps which is why muscle MOPAR's charge circuit wiring is weak.

All that said, with a stock system, you could spend $5 at Autozone for a length of 16 (or 14) guage fusible link and fit it in. Unless you are going to bypass the kinda weak stock system, that's that I'd do.