If anything err to the side of too much backspacing as you can always make clearance to the inside easier than outside or use spacers. The 275 tire would be under 11 1/2" section width but i'd still never trust published specs. These people don't own a tape measure and i've had to retub my car to fit a supposedly narrower tire.
There is also a big nasty bump stop perch in the wheelwell that can be cut down then the next obstacle is the leaf spring. On my rusty 66 Comet convertible i've moved the leafs 3" inward into a tunneled out frame and now there's quite a bit of room.
I've got 2 sets of 15x8" stock 60's steel wheels i've made up that are 4 1/2" backspace and 6" backspace. They are done but using centers from stock skinny wheels welded into 8" wide aftermarket wheels and could be made anything in between.
