Part I:
I'm going the delco route. I'll chronicle my experiences here for the common good, and because I like to hear myself type. Yesterday I bought NAPA part # RSE2134350. 78Amp alternator. It fits an '85 chevy truck w/ SBC. Comes with a groove pulley and fan. This is definitely not a direct bolt in, there's one big offset bottom ear as opposed to two smaller ears as on the bosch; but I went with commonality instead of ease of conversion. The parts guy said this was about the most common alternator around. We looked through the napa alternator book for twin-bottom-ear alternators, but none had the combination of cheapness, commonness, and ease of wiring. Oh, and I'll have to solder a lead onto a diode for the tach.
On the dead Bosch, It seems a screw that held on the diode pack or rectifier bridge or whatever the thing was, fell out and welded itself between some diode components that shouldn't be in direct electrical contact. Nice rebuild quality there, NAPA. I'm going to get a replacement under warranty and have it up for sale here in the future.
Part II:
I converted over this weekend. No cutting the stock stuff, or welding involved. Some flat bar, hole-punched garage door brace, and bolts and washers, and we were good to go.
Start by removing the lucas, magnetti, bosch, etc. alternator.
There is a 'lazy z' kind of bracket that goes from the head to the bottom alt. bracket that must be removed. I used the hole in the head for another brace later on.
When you remove this brace, and mount the small threaded ear of the delco to the stock bottom bracket, the pulleys line up exactly. I used the stock long mounting bolt, flipped it so the head was toward the motor, spaced it accordingly, and threaded it into the small ear of the delco. I also used the nut as kind of a locknut on the other side of the ear.
I extended the original jackscrew adjuster with a piece of flatbar. The adjuster didn't line up with the big ear (which is counterintuitively on top now), so I used some washers to space it accordingly. A 3/8" carriage bolt's square end under the head fits in the adjuster slot nicely. It also jams into the large ear's hole nicely.
The final brace was made from a piece of thin hole-punched garage door angle brace I had laying around. I cut this from an angle into a flat bar and ground down the sharp edges. The delco has a grounding/mounting bolt on the back which lines up with the hole in the head the 'lazy z' brace was originally bolted to. The two surfaces were almost exactly aligned, so a very stiff, thick brace wouldn't have worked. I ground two holes into a slot for the alternator bolt, and bolted the other side to the head, being sure to space the head bolt out with washers so it wouldn't bottom out. This brace gives the alternator some torsional support along the Y axis of the car that would have normally been provided by the large ear being mounted to something more substantial than the adjuster arm.
Wiring was fairly easy. The brown and yellow field wire went to the exciter terminal. I jumped the main power stud to the voltage reference terminal with a short piece of wire. Then hooked up the main power wire. I wasn't industrious enough to accomodate the tach, but I can live without it for now.
The truck idles much better than it ever did with the bosch. There's no slight red glow from the ignition lamp as there was with the bosch. So, whoever suggested my original alternator was knackered in my 'stumbling rover' thread was right. My original suspicion of grounding problems was unfounded, now that I actually know how the field / ignition lamp circuit works.
