Drilling rivets is one of those really hard things you just have to do with an older airstream (especially the two classics you have). My hubby, who manages a team that works with industrial control panels, just kind of does it without thinking. For me, it's suck my breath time. I don't mind the interior rivets, which are generally (for us) pop rivets--they have divots in the center, which make drilling easy. Exterior rivets are tougher. I tried getting one of those collars that are supposed to center the drill perfectly, but the collar I got was frozen. What ended up working was a tap to set a little impression in the center of the rivet. Get that little divot and the rest is easy.
One other suggestion--getting #29 and #31 drill bits is like discovering the existence of chocolate. These bits are exactly the size of the rivets used on the trailer--but they are specialty bits, so you'll have to search for them at specialty hardware places.
One thing--if you ever end up drilling out a big hole, don't get upset. A large rivet will probably fill it up for you (and dip it in a dollop of vulcam if you are concerned).
Sounds like you have two amazing trailers, can't wait to hear what you do with them! Just remember the mantra my mentor WBCCIer gave me: there's no way to irreparably screw up what you do to your trailer, so long as you keep to your vision and are willing to fix your mistakes.