This is not such an odd thing, I was going to post something similar in a day or two!
I have an old bike with English threading and a 70mm shell. However, I was just looking at swapping out the old with a cartridge. Not to try and hijack this thread, but is it possible to get either a 68mm or 73mm to fit the 70mm shell? If so, which one would be better?
As an alternative, could I just buy cups and spindle separately and "roll my own"? I would prefer the cartridge because I am trying to keep costs down on this project.
And if neither works, what can I expect an LBS to charge for facing and chasing the BB to get down to 68mm? (for those of us who are not luck enough to know frame builders).