I did the same thing with my uncle's 35 Remington Marlin 336 that was made in 1964. I found 3 boxes of unfired brass that were marked $3.25 per box, did the load development and found an awesome recipe that shoots .5" groups at 100 yards. 200gr Hornady over 35.5 or 36grs of IMR4064. Both loads shoot the same at 100 yards. Needless to say he was stoked to have the ammo since it's hard to find.I think is pretty awesome that you want to work up loads using the supplies purchased long ago.
I've got my grandfather's 30-30, and I can't wait to take a deer with it. I've got plenty of larger guns I could use, but to take a deer with my grandfather's rifle will be pretty special. I'm sure you will feel a lot of emotions when you take something with his bullets.