I'll throw in some input as I'm in a similar situation at 6' 4" and 290lbs.
The simple fact is the 67-72 trucks just have small cabs, but there is hope.
I drove my truck(daily) for several years with a 92 bench seat. It had a slim back, and it sat a bit lower. When I had the truck painted, and new interior done, I installed a 72 Cheyenne bench. The upholsterer used dense foam to build the seat. The seat looked great, and it was super comfy, but the driving position was terrible!!! I sat too high and too far forward. I just dealt with it the best I could, and eventually grew accustomed to it.
Here's how it looked when first finished.
Here's how I looked driving it!
A few years ago, I finally ordered a set of the Full Custom Fab brackets that lower the seat 2". But, the issue there is the seat sits lower, but due to the angle of the rear of the seat and cab, it actually slides it forward a bit also. So, even though the steering wheel positioning is better, the leg angle is worse. However, it's still easier to drive.
Here's some photos with the FCF brackets installed.
I don't have any photos of driving with the FCF brackets.
Now, I know you wish to keep a bench, so this is only for reference on the route to go with seats. I autocross my truck quite a bit, and a bench seat just wasn't doing anything for driving position, or keeping me planted to be able to control the truck, so racing buckets were a must. These Corbeaus have a very thin back, and sit lower. I have great positioning, and though my long legs are still wadded up, it's very easy and comfortable to drive.
I'm only mentioning the buckets to let you know there is hope, but it's not cheap. My suggestion is to look into a Snowden custom seat. They have formed cutouts and better slider brackets to position the seat better.
Here's a Facebook link.
https://www.facebook.com/snowdencustomseats/
Here's the AZ ProPerformance link.
https://azproperformance.com/product...at-upholstered
Another option is the TMI seats. The prices are similar. However, I've seen and heard mixed reviews about their products, so do your own research. I've sat in a couple trucks with TMI seats, and they felt great and had good positioning.
https://www.tmiproducts.com/chevy/tr...complete-seats
Jason Vannatta is a dealer for them.
https://www.vannattafabrication.com/
As for the calf strain and cramp, that's actually lack of muscle memory and conditioning. Eventually, the muscles will get used to it and go away. I've noticed if I don't drive my truck much more than short drives to town or work, then jump in and take a long drive, it wears my ankles and calves out!!