I'd say try it like it is first. My '69 Nova (8.2") used a driveshaft from a '72 (8.5") just fine, & vice versa. In fact I used the '69 shaft in the '72 for 100,000 miles or so. But I also saw where a guy at another site put an 8.5 in his older car (8.2), & he had to get a 1" shorter driveshaft. For measuring info:
Find a good driveshaft shop & they can shorten it for you.