700R4 GM in a Swinger?

Alright...I wanted to get some opinions on this. I was at a display for a place called Phoenix Transmissions while at the GoodGuys show in Columbus a few weeks ago, and have been swapping e-mails back and forth with their tech guys. I want to put an overdrive tranny in my Swinger, and was looking at either a 500 or 518, but I'm being told that a GM 700R4 would fit better with no floorpan mods, and they have amount adapter and all the hardware to make it a bolt-in process. They also claim that the 500/518 would probably require mods to the floorpan. I have been checking on tranny prices/torque converters, and, as usual, the GM stuff is easier to find and cheaper. The 700R4 is very close in size and mount position to my 904, and I don't want to beat the hell out of my floor to put a tranny in. Any thoughts?