FWIW, I raised it with 13" Girling gas shocks in place of the Rickman supplied 12" Girling oil shocks in about '77, still with 19" rims.
Now I have 18" rims and 14" Maxton shocks and a slightly longer swinging arm which which in total lowers the front a bit and raises the rear about as much as you can. The swinging arm is really there to accomodate a 130/70 tyre as opposed to a 3.50 triangular!, made in the original style round tube, not a box section as many use. The overall tyre diameter is probably about the same as the original 19" triangular!
With a Maxton front end set up for my weight (a fair bit more than it was in '79!) it is not 'too stiff' on the suspension, but it is fairly firm, got a harsh thud going through a bumpy dip at Gedinne each time, but OK everywhere else. I have only felt chatter at one meeting with it, end of season 2015 at Snetterton 300 going onto back straight, which after one click increase in front rebound led to going a bit quicker and some chatter at Coram and another click, fixed. Haven't adjusted it this year.
I am happy with it, but fairly sure I work harder than a Seeley rider, and since I am not as fit as most of them, I struggle a bit after the middle of most races. I typically see my best lap around lap 3. That is where tuning me, will help more than any tuning of the bike!
