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Really confusing! Your original post said you were replacing a THD 350 with the same...Where did the 700r enter the picture?
the yokes/t-cases will interchange but the th350 is about 2" shorter. unless its a different adapter/t-case. but they should have to shorten your shaft, which is easier (you could almost do it yourself with a cutoff wheel and welder) and cheaper sorry didnt think about this detail as being redundant as the 700r4 is pretty much the th350 but is longer due to the OD. the shop isnt lying but i would look for a driveshaft from a scrapyard or the donor the 700 came from.