Front driveshaft diameter

[Short 3/4]

My current truck was assembled from several parts trucks from 64-66. There were two front driveshafts in the pile, one 2 1/2” and the other 3 1/2. I picked the smaller one because it was in better shape. I’ve now managed to wear that one out, so am looking to build a new one. Since I’ll be starting from scratch figured I’d make it the correct size…so what would that actually be?

Thanks for any help
Tom