'67 B-body perches are at 44" center to center, I'm surprised you were able to get the springs mounted on that at all. Horrible idea.
A 1" offset kit from Dr. Diff would be your only bolt in solution, otherwise you'll have to cut and weld the perches. As far as tire size, it should go without saying that you'll be able to run another 1" of rubber per side with the offset kit. Without it you can run 275's with no modifications, some cars can do a little better than that but it depends on how you set up the backspacing.