The biggest factor in head room is probably the condition of your seat foams and springs. I never had head room issues in my fastback with the original bench seat, then I replaced it with junkyard 67 buckets that I recovered. I had the studs in the front holes because I'm tall, but I actually sat very low in the car because the foams and springs were well worn. I could wear a fedora or even a cowboy hat. Years later I had the opportunity to buy a freshly restored bench so I went with it, but I found in the stock mounting location my head was touching the headliner — the car was practically undriveable. So I made a pair of plates to relocate the tracks rearward by 2-1/2 inches (the bench seat mounting locations are sloped, so rear = lower). Now at least I can see out the windshield. Maybe in another 20 years the seat cushions will settle to a reasonable height. I think part of the problem was that the guy who had this seat reupholstered weighed at least 280, and I weigh less than 150.
My father's convertible had 1" wooden risers under the bucket seat tracks on the driver's seat. These were apparently installed by dealers if the buyer wanted a higher seating position. My father was 6'3" but he drove it like that, probably because the seats were beaten down (he bought the car in 1978), plus he weighed over 200lbs. I removed them when I inherited the car. Anyway, you could cut a riser strip on a slant to lower the back of the seat relative to the front. Basically, it's where your butt sits that sets your head height — the front edge of the cushion is just thigh support (since I have skinny thighs I like having the front edge higher — some people may find this creates too tight a squeeze to the steering wheel).
I think a solid riser would be better than a single spacer at one end, because if the track flexes in the middle it would jam. But maybe it's heavy enough steel that this isn't a problem. I have been fiddling with the tracks on an MG for similar reasons, and they will definitely jam unless perfectly aligned and square.
