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Thought I could help by looking at the cab mount kit I just bought from Dennis Carpenter but upon inspection I find they send me the wrong bolts. The ones I received are for a 48-52. But, look at Carpenter's and Mac's web sites and I think you'll be able to find the sizes.
What I do know is that I have some 1/2 by 1 7/8 course thread bolts that I am pretty sure came from the rear cab mounts (I know for sure they will work) and they fit snugly through the front mounts although a bit too long. All I would recommend is that you use grade 8 because of all the stress on the mounts.
I measured mine. 1/2" × 2 3/4" bolt with 1 5/8" shank. Mine are the same for front and rear. I would just order the kit though. I doubt you'll find nuts and bolts much cheaper at the hardware store. Cab Mounting | 1953-56 F100 - F250 | LMC Truck