So, I'm wondering if it is recommended to stay away from the one-piece necks...or is it just a case by case thing? Meaning, if you find a nice one-piece neck that is straight and the truss rods work from 68-71, it probably is a safe bet it will be OK for another 40 years ?
I don't have any experience with the 68-71s, but my 1995 4003s/8 has a one piece neck and it's fine (well, it is
now... and always was from the standpoint of the truss rods). I've run EXL-100s on my 4001s for a loooog time, and both have been rock solid. I suppose that an old Ric's neck could warp and twist just as easily as they might in an old Fender, and so same risks apply.
I might be restating what you already know, but the nice thing about most of the Rics is that you can completely remove the rods with no major surgery. That's actually how you adjust the necks, rather than torquing on the truss rod nuts like most other basses. In fact, tightening the rods that way can cause damage - and I think that's what Ilan was getting at about cracks and whatnot. Sometimes you'll see old Rics with big cracks between the fingerboard and neck up by the nut as a result of people adjusting the rods incorrectly.
The necks on mine are really pliable. I pull and resahpe the rods into an arc, use a "stretching jig" (table and ratcheting tie-down straps) to bend the neck to the amount of relief that I want, and then just reinstall the rods and snug them down. The rods won't influence movement of the neck, but rather hold the relief exactly where you (manually) set it.
Here's some pics showing the process on my old '78.
I use a clamp at the first fret to keep from popping the fb off when pulling the rods out. I cut a piece of aluminum channel strip to use as a guide for the rods, and tap them out from the body side:
Here's a shot of the s8 on the stretching jig:
So chances are, unless the neck is truly warped/twisted, you'd be able to massage an old Ric neck into shape...