What current rating for an alternator replacement.

I believe your '73 is a dual field unit, and if the outward appearance of the 90 amp alternator is acceptable, I'd go with the 90 amp version. For me, it's all about the output at idle. Many alternators can produce more amperage than folks need at higher engine speeds, but perform pretty poorly at idle rpm. With no power hungry accessories, the 90 amp will be all you will need while providing a little cushion.

Be sure to consider reading the information on Mad Electric regarding proper grounds, adding a fused or circuit breaker protected cable from the output terminal of the alternator to the starter relay post and doing the bulkhead and amp gauge bypass.

http://www.madelectrical.com/