...also depends upon the terrain you'll be riding; BiTD there wasn't the mania for the gnarly stuff that fashion dictates we should be throwing ourselves off today so the forks were more about comfort than technical, long travel, performance.
Myself, I don't have the slightest inclination to try and emulate Martyn Ashton every time I throw a leg over one of my bikes, hence 80-100mm is more than enough...
As for b*ggering up the geometry, I really don't think you'd notice much difference (and if you did all you'd need would be a shorter stem...).
I run 1996 Judy DHs on my 96 cindercone and it handles great.
I think you need to run old skool ATC length forks on the older frames, a modern 80mm fork will be longer than the originals with equivalent travel
Cheers
Adey