driveshaft Length HELP ASAP!!

Im going from a 8 1/4 and 904 to a 904 and 8 3/4 what is the correct length for a duster 108 wheelbase and a 904 with a 8 3/4?

what I got from this site http://board.moparts.org/ubbthreads/upload2/4563759-PartNoDriveshafts.pdf

is that my shaft is 49.90 and I need 48.96 is this correct?