Thank you so much I appreciate all the input.
I've been a captain on the west coast for 30 years then from Seattle to Alcapulco I've just been told that the river system is a whole different animal and I'm looking forward to the adventure
The river system is very different. It's a beautiful trip. However, if you only read or listen to the Captain John's of the world (captainjohn.org) you'll get a wonderful overview but a bit distorted view in some regards. Nearly all the waterways of the loop are used by commercial traffic with far greater size and draft than recreational boats. The exception is the Erie and Oswego canals. Air draft is more an issue than water draft. Yes, there are some areas you can't take your boat too, off the beaten path, but no areas that we'd choose to take a 4' draft to and wouldn't take a 5'6". A dock here and there that might be an issue but the dock beside it has deep water.
We did it in a boat that measures 69'4" and had no problem. We did select the marinas with greater space but it wasn't an issue.
Instead of open water, you're talking some open, some river, some of that winding. In some areas, you'll be traveling with large barges on your route. It is a whole different animal and you can pick up some of that feeling from Captain John and from the Loop Organization.
As in any boating though don't let one person overly influence your choice of boat. I read one book from a family that did it on a catamaran and recommended that for anyone. Someone else recommends a sailboat with no sail. I know a captain who took a 75' Hatteras through. Then don't overlook comfort as a factor. If you're use to one boat, you have to decide how you'd feel about another. Most small catamarans, the staterooms wouldn't work for me.
Seattle to Acapulco you have some long runs with nothing along the way. On the loop, you don't have much of that. Perhaps the Illinois and Mississippi between Chicago and the Ohio. One other huge difference is locks. Get use to them. There are a lot.