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Just swapped my 75's 400 with a 700r-4. No clearance issues, the only thing that is different is the tranny mount location and the length of the transmission.
I had to trim 2" off my driveshaft and fabricate a custom bracket for the tranny mount. They also sell these brackets, so you don't have to fabricate it if you don't want.
Definitely worth the extra hassle, having overdrive allows me to run 3.73's and still cruise 70 at 2100 rpm. And not to mention the gearing is much better than all them 3 speeds, hits HARD in second.
I replaced my TH400 with te 200-4R and everything bolted up without issue. You will need a new yoke and a way to hook up the TV cable to your carb.
Checkout http://bowtieoverdrives.com/index.shtml