i have a 1973 plymouth scamp and i switched the rear-end out with one off a 1975 dodge dart swinger its the same rear-end but with bigger brake bolt pattern but when the driveshaft was re-installed there is about 2 1/2 inch space where the driveshaft hookes up to the transmission. the rear-end is the same measurement as the original one. please help... thanks in advance  

How can that be....

 You removed which rear and installed what rear...

 If you removed a say 8.75 and installed a 8.75 then you should have no issue..but if not then there is the 2.5" diff  

mark the drive shaft into trans yoke to trans case seal as it sits,remove rear ujoint bolts and slide the driveshaft into the yoke until it bottoms out. mark again, now take these measurements and see the difference . you want to be no more than 3/4 inches from bottoming out in the trans case.also check the spring perches on the donor car as they may be located in a different location from the 73,its possible,other than that if your 2.5 inches from bottoming out in the case you need to either shorten the driveshaft 1 3/4 inches to 2 inches. my best advise would be to get the donor car driveshaft........  

on the 8 3/4 rear super there are 4 different cases and they all have different snout lengths. normally its not enough to matter.I run 3/4 on my spring cars before bottoming out  

Is it a 4dr? some of the early scamp 4drs were a diff wheelbase. measure the wheel base of both cars.  

ya probably just need the longer yoke from trans to driveshaft