I myself would rule out powder coating on any frame. There is no way to repair it afterwards so any scratch or chip becomes a problem. It's good for display bike only. And they don't need that much protection.
The insides of the down tubes wind up with the grease from the springs protecting them. There is a small hole on the bottom of each to allow water to drain out. The grease always ends up blocking this so they fill up with grease dirt water and gunk.
If you DO ever powder coat these must be cleaned out first rt the heat will allow that grease to drain out and mess up the job.
The frames were never clear coated so I wouldn't go that either. Just my opinion. Makes them look too good.
They don't really need any more protection than good paint. Easy enough to coat the insides of the down tubes AFTER they are painted with a solvent/grease mixture if you feel you need to.
I believe the original finish was actually a semi-gloss. Over time this gets a "patina" which is fact made up of many fines scratches.
I'm attempting the same thing at the moment. I'll be experimenting a bit more with a light application of steel wool to the top coat to see how close I can get. Not as hard as you might think because a lot of the frame is hidden/protected by various things and gets very little wear.
Probably the worst thing you can do is have a shiny black frame with rusty, faded, tarnished, scratched bits every where else.