Hi Glen. I too am 6'3" and have been driving X1/9s since 1977 so I have a lot of experience with trying to fit in the cars.
First, I would advise against moving the rear cabin bulkhead rearwards. That is a major chassis structure and moving it will require major surgury and may compromise the structural integrity of the chassis. Another issue is that moving the seat that far back will have your head right under the targa bar and you may find it pressing against the top of your head. This isn't a good idea as it could result in a spinal cord injury should you hit a major bump.
You mentioned your "old man's '74", is that the car? 74s (I own one) are the best for tall me. They have the tinnest seats and a smaller dash/console. The 1500 cars are the worst as they have the thickest seats and the widest dash and console.
As noted already, there are a number of easy tricks. A smaller diameter steering wheel, modifying the seat tracks to move the seat further back, changing the angle of the steering column, etc... are all productive. I have found that the seat is the most important thing to fitting in the car. Instead of moving the seat back make the seat thinner.
I cut down the thickness of the bottom seat cushion and modified the bottome seat frame so it was right on the floor. I had to cut away the return spring mounting bracket that is welded to the floor for clearance. I also cut some of the foam out of the seat wings to make the back thinner. These really helped free up some room.
In my racecar, I modified an aftermarket race seat by cutting the bottom of the seat out so I could sit directly on the floor. The seat back is only about 3/8th of an inch thick including upholstry so it afforded lots of room and was actually quite comfortable.
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