driveshaft problems

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 hooks up to the transmission. the rear-end is the same measurement as the original one. please help... thanks in advance