Either you picked-up a silly little mm somewhere or you have a unique fork, as the 1985-1992 forks are 40 mm. Typo by the tyro? Possible.
My plan for you:
ditch the early-model front end. Don't waste time or money trying to make it work. There's something better which is a bolt-on swap. Buy a good used set of 1993-2007 VMax stock 43 mm downtubes, sliders, two-298 mm brake discs and triple trees. Replace the steering stem w/All-Balls replacement bearings. Maybe new seals and oil if the forks are leaking. Read the warning about downtube condition, below. The 43's vs. the 40 mm tubes are 13+% larger in diameter, and the qualities of stiffness and resistance to deflection are noticeable compared to the skinnier tubes. My physics are a bit rusty, and I would have to search for the calculation formulas to compute it, but the larger dia. tube physical properties such-as resistance to deflection increase by a factor of more than the % of increase in diameter, and even more-so if the wall thickness is the same.
Buy early R-1 brake calipers (blue-spot or gold-spot) w/the 100 mm O.C. spacing, and HH pads. These will bolt to your stock 1993-2007 fork sliders, no adapters needed.
Buy a Kosman stock VMax hub rear wheel sized to 5.5" X 17" or 18". Buy two radial tyres (hey, he a Brit!) to for the modified stock rear wheel and the stock front wheel. I suggest Bridgestones, Metzelers, Dunlops or Shinkos.
With some judicious parts-searching, you should be able to buy all these parts for under $1000 USA to maybe $1300 if you want to rush things. Do not waste time buying corroded, rusty downtubes. New seals will immediately fail, and there is no fixing the rusty tubes, they belong in the scrap pile.
You will benefit in two important ways: you will be able to run radial tyres which will solve all-sorts of notorious VMax handling issues, and your front end will work much-better than what you have now. Progressive or Race-Tech springs and Gold Valve or RICOR Intiminators will upgrade your fork function even-more and should set you back about another ~$200 USA.
Adapting an R-1 fork or other USD fork will be very expensive, and your speedo will need work. Plus, your wheels won't match. Dropped triple trees are expensive to make the sportbike USD downtubes/sliders work on a VMax.
I know it will cause a difference of opinion w/people who have invested $$$ in their 40 mm stock front ends, but to me, when something else superior is readily-available, don't waste your time/$$. Go late-model front end and modded rear wheel and get radial tires, the bike will then be better than you, and at a reasonable cost.
Note in the pic, the calipers are FZR literbike, not R-1's, and I never mentioned the fork brace. That is a radial tyre on the front wheel (rear is a radial too, of-course), and a Kosman 17" X 5.5" modded rear VMax wheel, heavy, but very cost-effective. Think of it like a flywheel, storing and delivering energy.
