Footpeg to seat

I'm guessing the seat to peg relationship is shorter on the older bikes, but I don't know. I think you're on the right track with tall seat foam. And I know some of the newer pegs fit the older bikes. For example, the 2008 yz250f pegs were lower than the 2007 yz pegs and several of these ti yz pegs fit the older Hondas. You could try to mix and match.

I put DRZ125 pegs on my 77 RM125 and it opened up the riders compartment nicely as the stock were about 3/4 taller.

On my 79 YZ I ran later model YZ triple clamps (85-87) that had rubber mounted handlebar mounts and also were vertical instead of the old school in your lap laid over mounts.

You can do a bit to make all of the old stuff better you just have to be resourceful to make it work.

Most of the late 70's to early 80's bikes had a short seat-to-peg distance and short bar-to-peg distance. If you can find or fabricate a taller seat, it brings it more in line with a modern bike. Measure the distance on your 450 with a tape measure of piece of string, then measure it on an older bike, and you'll see what I mean. I think I try to keep mine at about 21" from the top of the peg to the top of the center of the seat, if I remember right.
Another thing on the older bikes that can make it feel cramped is the length of the headstay on the frame, or the bar clamps. Most older bikes have shorter frames than modern bikes, and the swept back bar clamps make it feel even more cramped. Again, take measurements and you'll see what I mean. If you can find a more vertical bar clamp like 450exc115 mentioned above, that helps too. I am 6' tall and I have had to put taller seats, bar risers (the 1 1/8" bar adapters work perfectly) and even turn the bars forward slightly on my vintage bikes to get them to feel anywhere near my modern bikes.

I compared the seat to peg difference on my KX250s:

Bike ------------------------------ seat to peg diff (inches)
2007 KX250 (tall foam) ------ 23"
1987 KX250 (stock foam) ---- 22.5"

KX 250's were considered tall even in the early 80's. I remember in the 83' or 84' 250 shootout, they gave the KX bad marks for handling because it felt 'tall' and wouldn't settle into turns as well as the other bikes. Measure the seat on a 82' YZ or 82' RM and I'm sure its less than 22.5"

A tall profile isn't necessarily a bad thing. I'm 6'2" and love tall bikes.