I'm 6'2 and 280 lbs, and have size 15 EEEE feet. I regularly squeeze myself in small places, for some reason. I drove a Miata daily for 5 years and owned a '55 Willys CJ-5 for a while.
With the truck cabs, you need all the seat height you can get. If your seat cushion is sprung your knees will hit your chin when you are trying to drive. As long as you have enough headroom and your knees don't get pinched by the steering wheel, thicken up that bottom pad, take a little out of the back, and you will be able to shift fine. I can drive my truck really easily, even wearing motorcycle or work boots.
The sitting position in an old truck cab is about like an office chair. Upright, legs straight out, elbows against ribs when holding steering wheel. Don't try to sit like you are in a sports car (low and leaned back) and you will be pretty comfortable.
