Go to here:
http://www.ktm495.mxbikes.com/Frame-KTM-1981-All.pdfThis will download the parts manual for the chassis.
On page 24 Diagram 03b, you will notice the head stem has a measurement of 'b' for the Frame part #1.
As per parts book :
(head stem length) b
=150mm for '80 mod 350/390
=150mm for '81 mod 250/390/420
=180mm for 495
Also to note is that there are two different measurements from the swingarm pivot point to the top shock mount (c to c). As follows:
350/390 mod '80 - a=250mm
350/390/420/495 mod '81 - a=270mm
Other differences occur in the swingarm lengths between all bikes also depending if they are GS or MX. Check the manual. ie 554mm,600.5mm,554mm,569mm,584mm.
Depending on which frame you have also determines the shock length as well.
Study the manual, and also read the remarks next to part numbers stating MX/GS or other information.
Triple clamps are offset differently from the '80 swept back mount style to the '81. Checked this the other day.
Hope this helps.