If he has 100 plus hours on them I wouldn't expect any issues with reversion then.

Yeah, as long as it's really 100hrs. Would there be any way to tell the difference between 10 and 100?

Not really. Compression, leak down etc. but... I wouldn't be afraid to ask the guy to do a comp test. He may dispute it all depends on how serious he is with moving the boat. If someone asked me to do one in advance I'd do it and document info. But that's just me. Some might say stick it in your azz. That be the case more than likely has something to hide or perhaps just somebody you might not prefer to deal with. If both engines are synchronized and sound good with no apparent knocks or noises then I wouldn't be to worried about it. You should be able to tell within seconds if the guy is straight up or not.

What year, length? You can purchase a couple near new or new 454/330's for a little over 3k ea. Worse case.

Being honest it would be good to know that that much cam on a stock 330 with stock exhaust doesn't revert water . Next I'm curious how the performance is?

Pull the spark plugs in the rear cylinders if he allows you to. if it has a reversion issue you will see the signs on the plugs.