There's no way a 396 is original, since they weren't offered in cars until 65. I'd like to know it got into an old truck, that seems like an unlikely combination. People usually go the easy route and put in a small block. If it's actually a 366 that would make a little more sense. You might yank out the 396 and sell it for enough to pay for restoring your truck. I'd think they would be really sought after in the U.K.

'65 C-10, 65 L-60, 65 C-80, 72 C-60 & and a bunch of old Chevy cars.
Never time to do it right, always time to do it over.