Rob, gotta dig in the memory bag here, but I seem to recall some of the early '60's Chevs, might have been Corvettes, routed the wires down the back side of the heads and then under to come up below the plugs. You could easily fashion the wires from a cut to length kit, just make sure you have a proper crimper. For the brackets, you might check with Year One, or one of the other Chev "restoration" part suppliers, they may be repopping the brackets that, I think, fastened to bell housing bolts. Once you look at it you should be able to visualize the routing. As for clamping the rest of the wire, I've used some of those nice plastic ones from Mallory for 2,3,&4 wires, Mr. Gasket makes some too, but somewhat lower quality. I've taken some scrap sheetmetal and cut some brackets of my own and fastened them to the clamps using the through screw that holds the clamp/separator together.

I hear ya' on the under dash wiring. A few years ago I had a '40 Ford deluxe coupe that some hack had wired. When I finally got tired of it I pulled the dash one day, and started cuttin'. The clown had used good wire, and the fuse panel wasn't too bad, it's just that he left too much wire there, and just wadded it in. After some judicious shortening, reterminaling, and routing it turned out very neat. Really only took about four hours, but some folks just don't have the patience I guess.