I'm looking for the same solution for my 75 Vega. Since I will be running a mostly street solid cam 406 with Hedman longtube hedders, I'm concerned about keeping it cool in traffic. I'm willing to think outside the box to make it happen. The LT1 in the Vega wagon is what got me started thinking about a larger-than Monza radiator in front of the core support. I made a cardboard dummy a few weeks ago to check out the fit.
I recently removed my front bumper and brackets. Measuring inside of the frame rails is about 28 inches, and measuring outside the rails, the distance is about 32 inches.
I plan to replace the u-shaped bumper brackets (and associated heavy components) with homemade flat brackets that mount on the outside of the frame rails, which should allow me to mount a 31 inch wide universal radiator (for about $160-180 from Summit).
I'm also considering modifying the core support to mount the radiator at a slight angle (radiator leaning back towards the engine at the top) that would place the cap inside the engine compartment.
I probably won't make up my mind on whether to mount the 31 incher vertical or angled until I'm actually installing it, but i am convinced that it will fit. Also the core of Summit's 31 incher is almost as big as an entire V8 Monza radiator, so the additional work to get it to fit, should be worth the trouble.
I'll post my progress when I get further along.
75 Vega hatchback, EFI 5.3L/t350 swap kit, street/strip, budget build in progress