...I've read this thread with some interest, as I would eventually like to do this very thing with my 1985 P8. It came with Rigida 27" rims; the rear rim got severely warped, and I had to play with the spokes a lot to bring it anywhere near true. Of course, this resulted in spoke tensions being way out of balance and consequently a lot of breakage (thank heaven for that much-maligned Helicomatic hub...naturally, all the breakage was on that side). So I unlaced the wheel and tried to carefully straighten the rim by wedging the rim between pieces of wood and just pushing on it. I didn't take out my 32-ounce ball peen and do the Trondheim Hammer Dance on it or anything like that. But in the process, the rim joint started to separate, and I doubt highly that it is possible to fix that satisfactorily, so I am about to try my hand at rebuilding the rear wheel using a new 27" rim (a Weinmann RM19; not the best, to be sure...I see that Weinmann have done the same as many other once-respected manufacturers and put their name on, well, what the Germans have been known to call 'China-Schrott' (think Grundig radios!)...but at 15 bucks, if I make a complete mess of it, it's not a big loss) as soon as I can get my paws on some 302 and 298mm spokes. If it works out, I'd eventually like to build up a 700c set, seeing that 27-inch is going to be increasingly difficult to replace. I've already measured the brakes; I've got the 4mm difference in reach more than covered (btw, 3/8 inch is considerably more than 4mm...it's actually very close to 10mm...3/8 inch=0.375 inch; 10mm=0.394 inch)...what I would ideally like to do (I know this will sound positively insane to many here!) is to somehow get hold of another Helicomatic hub and build up a 700c wheel on it (the bike came with a Helicomatic and, despite everything that has been said about it, I rather like it...an excellent idea that perhaps wasn't as well executed as it could have been...but perhaps would have been if it had been given a little more time)...of course, I'd need to recalculate spoke lengths to compensate for the difference in rim size, and apparently it wouldn't be as simple as using spokes that are 4mm shorter...
...I'm not necessarily one to run from a challenge...