Has anyone narrowed the rearend of a '64 Bonneville to get a deeper dish wheel on the back? If so, what was the max it can be narrowed with out having to to change any brackets? And what is the least amount of backspacing it allowed you to run?

With such a wide Varity of rims.  You 1st should figure out what rims you would like to use and contact manufacturer on what backspacing you might  be able to order the rim in.  Without changing brackets or suspension.  Measure off the frame as it sits now. You can order a new rear sub frame, need to go with the much safer 4 link  and you can stuff any size rim N tire you want under there.  Good Luck and keep us up on your progress.