Alternator How Big? AMPS

[Fatolbaldguy]

I am just about finished installing my air conditioning kit into my 292 65 3/4 ton C20. Some hoses still need to be fitted and some holes in the firewall need pluging.
On the test drive I discovered that the tired old alternator can no longer keep up with the load. It barely managed before I put AC in. I realize that I need to swap in a new high output alternator but how big? How many amps? How do I determine whats best. I'd rather not take it into the shop since this project is way over budget and getting worse. Thanks