I'll try for an omnibus reply.
A few commercial sources for those that think this is just old-tyme stuff.
Yes, I'm (hopefully) redoing some worn out pivots - or eventually I am - I may need to keep abusing them for another summer to get work done, but if I can do a bit at a time and get the worst ones done, it would help.
Wes has the basic idea down, and one can buy exactly that from several places, but I see no reason (thus far) I can't put it together myself for about $1000 less.
Hole size ranges from an inch or so (too small for most commercial versions) up to 3 inches. It may actually be that the pin in the hole is an inch, which means that the bore for the bushings (MIA, or I'd just drive out and replace with new) is bigger - the smallest ones are for the steering linkage, and lord knows what some idiot did, but there's no trace of the bushings on those parts. Of course the loose pins have beat the heck out of the holes. The big ones are either a 2 or 2.5 inch pin with a bushing for another half inch diameter. Those 1/4" thick hardened steel bushings were worn right through and wear continued quite some distance in to the base steel.
George's book link is good, and he's got the right spirit - complicated and dangerous, that's us. The main difference with the newer systems is that the moving cutterhead is generally replaced by moving the whole boring bar through bearings, and setting the bit in a hole in the bar itself. This would seem to allow a thicker bar relative to the size of the hole, and that keeps deflection down. I don't think any of the modern ones try to run a pair of bits, either - getting them matched precisely must have been fun.
The mag drill by itself places a lot of dependence on getting its mounting perfect. At least one of the commercial bars does use a mag drill as its drive (and feed, perhaps...)
Bridgeport head as drive and feed is not a bad idea at all, though I'm willing to trend cruder. It limits to driving from one end, but that is not an issue for backhoe parts as far as I can tell. Some of the commercial bars tout flexible drive options for getting into more difficult situations, but the hoe (and loader bucket) are comparatively accessible, on that basis.
Radial drill press - I've had one on my list since I first learned about them, but haven't gotten one yet. I'll up my shopping effort.
Age and ugliness - Hoe is a ford 4500 industrial TLB, only 40-41 years old. Many joints and pivots are severely ugly (in the more important mechanical sense) through some decades of no grease, at a guess. I might have continued shopping had I known better at the time, but this was also less than half the price of the next contender when I was shopping, and a considerably better machine in the sense that it's a hoe built on a hoe platform (27gpm pump!), not a tiny hoe stuck on an ag tractor. I've certainly made it pay for itself - I've also wished it was bigger (it's a 13 foot hoe) which would have surprised the heck out of me when I was shopping and thought it was a bit intimidating size-wize - that was before I started stumping hardwoods in bony gravel. The paint is a crude job, and is has scars and bad welds (many not mine, some mine - mine are at least not the worst on it), plus some big plates that the previous owner was fond of tossing over cracks. My old welding instructor would have looked at those and yelled stress riser, and I agree, but have not had time to go over them and "fix them right". I do at least try to make my repairs somewhat better designed. The buckets both need work - I have a strip of old road grader edge my brother donated to fix up the severely worn front bucket - the hoe bucket needs teeth - well it needs shanks, too, as the "teeth" it has are actually terribly worn shanks that were dug a long time without teeth on them. Actual hard, sharp teeth would make root breaking about 9 times easier, and that's the key to stumping - but at this point I have most of the stumps out - the road is built, etc. Several cylinders are scarred, and most could stand to be repacked, but time is finite and work to be done is infinite...
What I really need is some sort of tractor restoration nut in dire need of a project, but that sort of thing doesn't happen, or they get terribly upset when you take the newly refurbished, tight hoe out and scratch the paint on it, or get it dirty.