Looks like the old cut&rotate is the best solution. I know you didn't want to hear that... but there is a local guy who will do it affordably and well if you gut the housing and pull it (no mean feat on an 80), and tell him EXACTLY what your angles are and what you want them to be. If removing the caster bushings works great (but doesn't steer great) I bet you could just take your measured caster vs. factory spec and he could make it happen.
Heb Dduw, heb ddim; Duw a digon
I'm that gun-totin', farm-raised, evangelical, pro-environment, OHV ridin'/drivin', Southern civil rights pro-labor Liberal yo' momma told you couldn't possibly exist.