Odds are great it was the original engine @ 86k miles so . . . great find!
That would make it a 500 with the later open chamber (low comp) heads.
But then you already knew that since unlike most folks, you [Search]ed first, and posted second. :thumbleft:
I know this isn't much help, but since you haven't had any other Cottonmouth comments, I will throw this out there anyway. A buddy of mine had a 13 footer like you elude to, that had a 351W on it. It must have been built right, because it ran like it. That hull did not have the motor (prop) well cut out of it, so the rigging was taller than most of what you see. Consequently, it felt quite 'top heavy' to anyone not an experienced airboater. Unfortunately, his wife never got used to it and over time he sold it because she said it was 'too tippy'. :?
Along the way, cutting out that well has been debated here at least once. I'm sure you can find it knowing how to look. I don't really have a strong opinion about that either way, although it seemed to me like a marketing tactic more than a functional improvement. Were I going to rig one with a Caddy for four people, I would most likely remove it because doing so would help relieve the 'tippyness' that may make Mom and the kids a little nervous. It would also afford you a little more elbow room when building your rigging.
If it is within your means to do so, I would not have any debate with myself about ordering a Marty Bray hull and would do that asap in order to minimize my delivery time. His hulls are as stable and as well built as any that you will find (<-- likely an understatement there) and not gain an excessive amount of weight while attaining that end. The only question I would need to answer before I pulled that trigger would be "Am I going to stay DD for the life of the hull or might I add a gearbox later? The reason I point this out is because if you think you might add a gearbox you would want to opt for at least a 14 or maybe 15 footer simply to increase your safety and comfort level while toting your family. When you are toting four people with a gearbox anything smaller starts getting cramped and sitting low enough in the water to potentially be a concern.
Being new to the sport, you are likely unaware of this, so I will share this in the event a new hull is not in your cards. In the beginning, Marty Bray was the man who made the original River Master hull molds. Dick Hoffman made those hulls in Inverness (using those molds) for years. They were known for their stability and strength, and were debatably the standard for 'glass hulls for a number of years. After Dick passed away, the molds were sold and ended up in South Florida. To my knowledge they never went back into production. Somewhere along that timeline Marty Bray made new molds with improvements and modifications to that design and has been making them ever since. The point being, if you are looking for a used hull and can find a River Master that is in good or restorable condition, that would likely be the next best thing to having a Marty Bray hull. I have two of them, so can attest to all that first hand. You don't see many of them for sale anymore but they are out that thicker than old Caddy's with 86k miles, I am sure.