7.25" to 8.25" driveshaft?

Now that the 8.25 rear end is in, I measured from center of u-joint cap on the front to the center of the rear end's yoke and came up with 51" (rear axle on jack stands). This seems like it can't be right. I am measuring with the slip yoke about 3/4" out of the tail housing - that is to say that I see about 3/4" of metal extending past the rubber seal. Or is it supposed to be from the end of the metal on the transmission tail?

Also, I picked up a heavier driveshaft from a junkyard Cordoba with a 904 and 8.25" rear, thinking that it would be a better match (bigger diameter tube). But would you believe that when I had the dual exhaust done, the guy ran the pipes very close together using the stock driveshaft as a guide and the bigger one won't fit between them! Is it worth re-doing any part of the exhaust system for a beefier driveshaft?

Thanks!