I did this exact trip from Michigan to the Bahamas spread over a couple of years. I sailed for many years out of Holland, on Lake Michigan. I had the boat trucked to Monroe, in the southeast corner of the state. From Monroe, I crossed Lake Erie and then into the Erie Canal. Then down the Hudson to New Jersey, where we spent a couple of years sailing out of Morgan, on Raritan Bay. After that I took an outside run to Norfolk, where the ICW begins. From there we went inside all the way to Fort Lauderdale, our jumping off spot for the crossing to Nassau, where we are now.
There are a couple of limitations that your 70 footer will impose. The overhead clearance on the ICW, which is what I took for a major part of the trip, is 60 feet. Controlling depth is supposed to be 12 feet but I had some close moments with my 5 foot draft. You'll have similar issues with the Erie Canal if you choose that route. I don't recall the controlling depth, but it ain't much. Masts down is a given. Length won't be a problem. I saw a cruise ship during my passage. It was specifically designed for the canal and does a circumnavigation of the eastern half of the US on a regular basis. I can give you a lot of particulars if that route is of any interest.
It was a great adventure. We're thinking about getting a bigger boat and my wife, who is a novice cruiser and not a brave soul, did the Maryland to Florida legs with me and says she would love to do it again.