Re: Mississippi river long distance cruise
I've done several long distance river trips, the longest being 600 miles and 13 locks from Pittsburgh to Louisville. That took 4 1/2 days in a 21' Sea Ray cuddy. The 3,200 mile distance I cited for your trip is quite exact. It came from Mississippi River navigation charts, something you MUST have on your proposed trip. They can be downloaded as .PDFs here:
Upper Mississippi River Navigation Charts
U.S. Army Corps of Engineers New Orleans District: Geospatial Data
Since it took me 4 1/2 days to go 600 miles, I think a reasonable estimate for 3,200 miles would be at least 20-23 days and that's not counting any layover days. Your estimated cruise speed of 35mph is not realistic. Plan on around 25mph.
The other route to the Gulf you have mentioned is via the Tennessee-Tombigbee Waterway. That would cut off many, many miles from your proposed trip and mostly eliminate the fuel range considerations. However, there are a lot of locks and several have notoriously long delays, but almost all recreational pleasure boats heading to the Gulf take that route.
Do not attempt such a trip on a shoe string. Research and then plan, plan, plan. I highly recommend taking at least a couple shorter trips to check your and your boat's capabilities. If I understood your posts correctly, you say your boat has an 18 gallon tank and that gives you a 300 mile range. I find that unbelieveable. My Sea Ray cuddy has a 5.0L Merc and Alpha 1 outdrive. It has a fuel tank with 48 useable gallons and I plan on 140 miles MAX between fuel stops. That gives me about a 25 mile reserve. Those figures have been validated many times.