the walls have some sort of a curve in all 3 directions... outer corners are slightly lower than the center of the wall on the left (off about 1/2"-1") the left and back wall's bow outwards in the middle (off about 1"-2")... most of this is from warping over the 80 odd years the garage has stood.
Actually, they haven't warped so much as they have been thrust out by the roof rafters pushing out on them. They bow out more in the middle as they aren't constrained as much as in the corners, and there are no ceiling joists holding the walls together at a fixed distance. All you have is a couple collar ties.
the amount they are off is relatively insignificant, so I wont be bringing them in with a come along. I merely want to cross brace so they don't go any further out of whack and to give myself places to hang additional lighting and possibly an air hose reel
If you're talking here about running some ceiling joists from wall to wall that's a great idea. That will keep further spread from occurring. Don't just nail then in place, use metal straps to secure them to the top plate of the wall at each end. If you want to use any of the space above them for light storage use 2x8's at 24" o.c.
The in state manufacturer of polyiso who actually has an R-30 rated 4.5" foam board wants a price per foot that would make my great grandchildren cringe... and wont sell to me direct anyways... instead I'd have to buy from one of their distributors in Mass to have it trucked back to Maine

So it looks like i'm going to be going to have to go with R-19 fiberglass mat like I did with the walls since nothing else is will really work at the shallow depth I have. I don't want to screw more wood to the existing beams just to get the depth too much work.
I am NOT doing spray foam/batting... not only is it expensive, the companies around here that do it ****.
I'm open to other suggestions/alternative solutions however they have to be less expensive than my left arm and right leg.