____Driveshaft Question??

Ok,

I just changed the rear end in my '75 Dart Sport. It's a 318 904 a/t that 'HAD' a 7 1/4 rear (SBP). I changed to a 8 1/4 rear (BBP) that came out of a '73 Swinger. The the driveshaft is to long and does not fit into the rear. I measured the driveshaft from end cap to end cap and it is 52 1/2".

So my question is, is the drivehshaft longer in cars with a 7 1/4 versus an 8 1/4?

Thanks for your time and looking forward to some input!