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Generally the stock driveshaft would be 29 7/8" (center to center) or a bit less I just checked Dennys and they require you to supply a measurement so I'm going to guess there's a bit of range and the 29-7/8 is more of a maximum, maybe 1/4-1/2 shorter could be ok with the slip yoke
The 700R4 also had a couple of versions that were about an inch different in length, chances are you're using a 30" long one rather than the "vette" one
Really, best to get everything in and then install the yoke and measure, you don't want it bottoming out or hanging out too far
M