The 550 and 750 forks had caliper mounts on both sides, so double calipers could be mounted. Also, the fork tubes were 35mm, instead of 33mm like the 400.
I've done them both ways, if you've got someone who can and will weld and fab up the right fork lower its an option. But all you're doing is adding weight to an already over stressed undersized fork with limited damping.
Or, you can take a set of 550/750 forks with a 450 triple tree (I believe thats the right one), and then decide what front wheel choice you want to use. If you choose the 550/750 front hub and rotors, your going to have to lace it to an 18" 40 hole rim, that way you keep the 18" rim front and rear like a stocker. I've never put a 19" wheel on the front so I wouldn't know how that works out. I've done this conversion and when you throw in all the extra weight that comes with it, it gives the bike a feeling of more stability, and it dampens which the stock unit doesn't, but its also mucho heavier. Bigger fork tubes, brake rotors and hub.
Oooor, you can use the 550/750 fork tubes and 450 triple tree with the 400f front wheel and rotors with the 550 front axel. There's a little modification to the caliper hangers to get the correct height for the brake caliper in relation to the rotors, as the 400f rotors are smaller in circumference than the 550/750 arrangement. Then you have to shift the wheel to the right about 8 or 9mm (to center the hub in the forks) and everything should line up. Should, its been a while. And regardless of your choice, ya still have to slide the tubes up in the clamps to get the proper ride height.
Of the two arrangements, I preferred the the 550/750 front end with the 400f front wheel. Ya don't have to lace up a 18" to a 40 hole rim, it looks a little wonky with a 40 hole in the front and a 36 hole in the rear, but that's just me. And its a lighter wheel/rotor combination but with the added stiffness and damping of the larger front fork.
