An original ST/400 racer......WOW, what a find! BUY IT, BUY IT BUY IT ASAP! I can't believe how good of condition it is in too. I can only imagine what stories that machine would tell if it could talk.
Again, that is an amazing find!
As far as the 1.25" axle conversion goes, I would say that it is very easy and straight forward. Whipper's jig for the conversion makes everything a breeze. I just did my green poly body Attex and it was a very easy process. Keep us posted on the build when you get it home.