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You should be able to get a 12.5" wide in a factory one, the leaf springs are going to be the limiter.
Are you sure Coy makes that 20-X14 in our bolt pattern? I only see it listed for F-bods and Stangs.
With the leaf springs in the stock location, it's like you were saying "A standard stock width truck" as far as tire clearance goes. As Chris said...the leaf springs are the limiting factor. It looks to me that the amount those are widened, they are back about flush to the frame. If you want a low-buck way to fit BIG tires (18" or so wide), you can remove the spring hangers from the frame (notch the bottom) & re-install the spring assembly on the inside of the frame, with some wheels with a fair amount of backspace, your all set. A rear cut down or One from say, a Camaro may make the whole thing look "more correct", now the frame is the limiting factor, not the springs. What the previous owner did was really just the easy, cheap part of the job & nothing else. The bummer is if you don't want the big tires, it's allot more work to "put it back" than it was for the guy who cut out the floor. Mine are widened about an 1 3/4" (back to the stock placement of the springs) which allow for my 13" total width tires with reasonable clearance. Lorne