I'm 6'5" (in the morning, I tend to shrink a little by late in the day) and also have problems fitting into my '52 GMC's cab. My solution was to install a 2nd row seat from a Chevy Astro van. The seat back is thinner that the stock truck seat, giving about 1.5" more room from the dash to the seat back. The "how I did it" is posted here.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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