You might consider posting/searching over on El Camiino Central where there are a bunch of '64/65 Elky owners sharing info. This resurrected thread has some info - look at post #11 & 12 - 1965 El Camino max. wheel/tire size - El Camino Central Forum : Chevrolet El Camino Forums Like Dave & Em have said, getting a baseline of your current wheel backspace, tires and clearances and then comparing to your chosen wheels & tires is the easiest. Another approach is to cut a plywood circle with axle cutout & bolt circle accurate to the center and the OD matching the new tires you want to run. Bolt a couple of long carriage bolts in place at a diameter 1/2 way between the wheel OD and the tire OD to match the tire section measurements to show widest spread. It works pretty easy for the back, with the front creating some challenges with the lock to lock positions but it will give you an idea. Or if you've got extra $$ laying around there's this - http://www.summitracing.com/parts/php-01201