There are several folks out there building higher output
alternators based on the old Delco Remey 10DN Alternator (around 100 amps) When you start getting around a 100 amps, you've reached the limit of what a single belt pulley system can handle. I speak from experience as the previous owner hung a 105 amp Lestek Alternator on our boat and belts don't last long.
You could get the standard externally regulated Delco Remey Alternator rated at around 65 amps for around $65 from about any alternator repair shop. I bought one this summer as a backup to the two alternator system on our boat (one for the starter battery, one for the house bank, both small case Delco models.)
Ditto on what Cam said about standard automotive regulators not charging deep cycle batterys well. Even with your existing low output alternator, you would see an improvement in your battery's health by investing in a smart charger regulator.
I don't know anything about the API alternator, so can't help you there.