I feel you pain Lead, we have two dogs, and if something happen to them........ Good luck with the surgurey.
BTW, if you run 345s on 12" rims they are not wider then 335s on 13" rims. I have put Project Prodigys 345 / 20s under Kevins leaf spring mini tub car and have measured section widths. The backspacing is differetn for the IRS then kevins live axle, but if it were the correct backspace they would fit.
Look at the following links and specs, this shows in Pirelli's the the 335 and 345 the same width on different rims, and shows Michelin's to be only a 1/4" different,
What will be most critical will be pushing the tubs in as far as they can go. Ordering the wheels and tires first, then mocking them in place to measure for the rear to be sure it is exactly the right width
http://www.tirerack.com/tires/Spec.j...e1=yes&place=0
http://www.tirerack.com/tires/Spec.j...e1=yes&place=0