Gentleman,
I have a Pre 64 Mdl 70 action that came off a rifle originally chambered in 308. Can it be used for a 300 H&H without a ton of work, or is it just not financially feasible? I know they only came in one action length and the conversion for say a 308 Win to 30-06 is pretty straight forward, but I don't know what is needed to make the 300 H&H work.
John

Find a magazine box & follower from a .300 H&H and you could in theory machine the action for it to fit since all pr-64's are the same action. In addition to paying for it, can your gunmaker of choice do the work? There's a lot of metal to massage to get from a .308 to a .300 H&H that functions correctly.

The easy button is tripping the .308 and finding an original .300 H&H to your liking.

Sell the 308 and get a new rifle. Making a pre 64 length H&H is not all that hard, but does take time, and labor is $100 an hour.