Look at the back of the cab on the 08. You will see it is extended at the back of the seat area to make room for the aforementioned thickness. The cab on a 65 c 10 is a box. Straight up and down. Most likely if the seat goes in, you wont be able to fold the back down to the point that it is comfortable. likely will be straight up and down.Also will end up way too close to the steering wheel.Newer trucks are more ergonimic, older trucks .....not so much.
Also as mentioned, width will be an issue also.
I would recommend fixing the seat you have
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