I agree about .001" interference is all you need. Alternatively, you could take it down to a near slip fit and use something like Loctite 609A retaining compound to hold it in. If you work it down to .001 to .0015 interference, putting the bushing in the deep freeze for about an hour should shrink it enough to slip fit, then expand into place as it warms up.
I know all this is too late to help. Sorry. I've seen a couple of these pics too. Old metal don't help (metal ages, in use or not) but a lot of these bushings are coming from supply houses that have pretty wide standards on parts like this, and verifying dimensions before installing can save some hardship. Last year I was converting GL1800 rear shock assemblies with the hydraulic preload adjust for use on Road Stars (Google "Road Wing") and the bronze bushings we used from McMaster-Carr ranged from pushing out when you put the center bolt in to cracking down one side when you pressed them. A little attrition on the stock, but nobody ever trashed a surrounding part. The wall of that cam is really thin, especially at the relief cut behind the threads, so you are pushing more internal stress to that thin point as you insert the bushing.
Sorry for your troubles.
I do have a suggestion though. With the Pamco, that end of the cam will be a low stress area. There is no more ATU inertia to cope with, just a cap device. You might consider slotting that crack out with a good radius at the bottom of the crack going inboard. I'd consider carrying the slot all the way up onto the major diameter beyond the threads where the cross section steps back up. I'd do it on mine if I had your issue.
