You have a Cummins 6B, right?
The Cummins uses a Delco as its OEM or maybe a Leece-Neville and has a 2" single foot mount. These are available at up to 180 or so amp ratings with connectors for an external regulator which you will need for good output. The Balmar ARS5 regulator would work fine.
So with potentially (and I say that because engine rpms and alternator temps will reduce this) 180 amps of power at 12 volts then you have 2,000 watts of power generation available.
You obviously understand about the belt wrapping solution which you will need with 180 amps of load.
A second alternator will be expensive and it won't be a 120V unit for the reasons stated above- rpms and frequency. You could fabricate a mount for a second alternator and drive a 12V, large case, high output alternator. Balmar makes them and maybe Leece-Neville as well. But the fabrication work, the alternator and the regulator will cost several thousands of dollars.
You can buy the standard case, 180 amp Leece-Neville alternator on line but it will take some research to find the right one that fits your Cummins mount. Or you can pay more and buy the alternator from Balmar. You can buy the Balmar alternator or regulator from Defender or similar.
Come to think of it, I would buy a Balmar alternator and regulator. They understand marine applications and can get you the right pulley to work with your serpentine belt. Their alternators put out more real power than even heavy duty truck alternators such as the Leece-Neville.
But do you really need that much power. Are you doing this to run DC loads at anchor, in which case running your propulsion engine at anchor with a light load on it isn't a good idea.
Most power boaters can run their at anchor loads from a decent sized bank of batteries and then recharge them while underway a few days later. With the standard Hitachi 80 amp alternator on my boat, I get along fine this way with only 200 amphours of house battery capacity. I have never anchored more than 2 days at a time and my cruising time of 4-5 hours is enough to recharge fully afterwards. If I needed more output or less cruising time I would go with the Balmar alternator and regulator, but so far haven't needed it.
David