I'd really appreciate seeing what you have. I've done some Internet research and found that there are quite a few systems, from simple steel plates that could potentially bolt right up to the E450 seat mount, to whole hog swivel recliners that I think are designed for Class A motorhomes.
Like you, I'm worried about whether there is sufficient room to swivel the seat. However the simple steel plate swivel is so inexpensive that a failure wouldn't cost that much if you just did the work yourself and bolted it onto the factory mount. This approach would raise the seat height by 1 and 1/2 inches, which will probably make my wife's feet not touch the floor (but then she's complained while driving that she's too low and needs a cushion).
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