Been a long enough time since I owned my '65, let alone changed a taillight bulb in it, that I don't clearly remember the socket details of the inboard lamp. But I want to say there's some aspect of it that makes this harder and more complex than the simple socket swap you have in mind.
Forgot to mention earlier: ceiling-white paint will very likely cancel an issue (if there is one) caused by the different focal length of the R5W versus the original 1095.
More detail:
Clean the lenses in hot soapy water. Stuff wads of masking tape in the bulb holes (or remove the sockets if they're the removable type), clean the reflectors with alcohol, then paint them with Ceiling White paint (specifically! Not just any old white paint; go to a paint store and get Ceiling White), which is practically ideal for the task, even if the original material on the reflector bowls was more of a silvery stuff. An example is 'Ceiling Bright White' from Sherwin-Williams; they'll mix it up for you as a polyurethane-based paint on request. It's water-based, so it won't stink up the place and it's easy to apply and clean up. Don't worry about the surface gloss (or not-gloss) of the ceiling white paint; surface gloss isn't the thing in an application like this, as it is in something like a headlamp.
This paint has 90-some-odd-percent reflectivity of incident light with broad diffusion. You might take a look at this vid for a demonstration; he takes awhile to get to the point, but he eventually (3:24) he starts to get around to it, and the difference is pretty obvious even with a low-quality camera. And he just used regular white, not ceiling white; regular white looks dishwater-grey next to ceiling white.
I did paint the lamp cups bright white when I rebuilt them. Not sure if it was ceiling white specifically, but it is was a really bright white. I'll give the R5W bulbs a shot first. If they're close enough, great. If they look off, I'll try replacing the socket since it's easy enough. Plus, I already have some extra sockets from some extra parts harnesses I've got.