Tranny confusion

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Hi all, new to forum,Im currently redoing 66 C10 Fleetside longbox. Purchased a rebuilt 283/275 hp I want to put an overdrive auto trans behind it. After much reading I am leaning towards a 200 4r (rebuilt and beefed up of course) but tranny shops in my area are all trying to talk me into a 700 r4 saying a 200 4r won't stand up, this seems to be contrary most of the info im reading on this and other forums. Should I keep trying until I find a shop that will help me or go with the 700 r4. Thanks for your time.