This is Jim on his 1975 SST 440 F/A. He mounted a 121" Polaris track under it, and was kind enough to send me the pictures to help you all do the same.
Go get a 121" Long, 15" Wide Polaris Track. You can usually get them like new pretty cheap because a lot of guys buy a new polaris and take the stock track off right away to put on something with bigger lugs on, leaving you with a brand new, but still much cheaper track.
You will also need 6" drive sprockets that match the same pitch as the track you're using. If you are using biggers lugs, you may have to go with smaller drivers.
The track will fit in perfectly with the stock suspension and drive sprockets, the only modification you will have to make is to slide the idler wheels back 1 hole so you can tighten the 121" track. Good Luck and Happy Sledding!
I tried mounting a newer Bombardier track on my SST as well, and it took quite a bit of work, but worked well.
The snoflap will no longer fit because of the track and suspension extension, but can be worked around.
We had a few problems installing the new track. For some reason there was way too much slack, so we had to redo the suspension by pulling the actual slide rail back 3 inches.
How to mount a 133" Polaris track on your 75/76 Sno*Jet SST or Astro SS
There really isn't much to it, just do the same as pictured above, use polaris 7 tooth drivers with the 133" track.
You'll need some brackets made to increase the clearance under the tunnel, as well as extending your slide suspension and adding another set of idler wheels at the end.