Well, as the OP of this thread I guess I feel somewhat compelled to comment on the wealth of input that it generated. One thing's for sure...I think I definitely got my money's worth!
A few observations:
I'm inclined to believe that an adj. reamer can work. However I think there are certain requirements necessary for this to happen. Among these are: a quality, sharp ( preferably unused ) brand name tool, an extended pilot to the drive side race and ( most importantly ) large amounts of patience and dexterity on the part of the operator to not rush the operation. Certainly this process worked for Thunderlizard. His bike's bottom end doesn't make strange noises and does make good oil pressure.
I tend to agree with Ignoramus regarding reamers though: "they want to follow an existing hole". So if the alignment isn't too far off to begin with, you'll probably be alright. The use of "shell reamers" as mentioned by CBS may be an exception to the above rule as well.
As pointed out by Nick, if using a mill, indicate off the parting face rather than relying on the timing side outer surface to be dead flat.... good info. He also makes mention of the fact that the hole (for the bush) in the timing side case may be knackered from 50 yrs. of use and abuse. This is definitely true of the hole in the case of the '70 engine I'm working on right now. I suppose it could be cleaned up with a light "kiss" from a cutter as long as you're dead sure of concentricity with the drive side bearing race (seems to me this is one place where an appropriate sized reamer might be ideal ).
Nick's comment that these bikes were basically the Fords and Chevys of their era is worth remembering. They employed good engineering practice for mass produced machines that probably no one working at BSA ever dreamed would still be on the road +50 yrs. later. As such, they don't require that a person be a "rocket scientist" to work on them when competent mechanical/engineering skills should be more than adequate. This leads me to seriously contemplate giving this operation a go on the mill myself. Thanks to Ray Elliott's post, if I manage to go through several bushings before getting it right at least I know I'll be in good company!
I also still feel that the end feed bearing conversion is a good way to go. But as I mentioned early in this thread Ed V is the only shop doing it in the U.S. that I’m aware of and he’s got a fair back log of engines awaiting that conversion.
Regarding EI, I don't have anything against it....but there was that ONE TIME when I fried the primary on my A65 and was still able to successfully complete the 100mi. trip back home by virtue of/thanks to the points/battery
system; of course this was before cell phones, so there really wasn't much alternative….
Many Thanks to all who have contributed their knowledge and thoughts....as always, very much appreciated.