The cutting of the springs isn't what shortens the fork, what does it is the spacer you use (pg. 3, last pic, the bronze bushing). That bushing prevents the forks from fully extending, hence making them shorter, and the longer the bushing, the more the fork is shortened (what you're doing is pushing the damper piston farther up in the upper tube). There should also be a small top-out spring where that bushing goes. It's there so when the fork tops out it doesn't slam the damper piston into the bottom of the upper tube.
When you push the damper piston up in the tube, the main spring will stick out of the top more and that's why you cut them. You don't have to cut them, you can cram the whole spring down in the tube but it makes the fork stiffer because you're essentially running with a bunch of preload.
I used this method on my 550 fork, but with a small twist. I used a longer top-out spring instead of a bushing or spacer. The reason I did this is there are damping holes down near the piston that a bushing would have covered up and I didn't know if that would be a problem. I found some springs at Ace Hardware that were the same diameter as the top-out spring, but it was about 4" long. I cut it down to 3" for about a 2" drop (the top-out spring is 1" long). I cut a corresponding amount off the main spring and called it a day.
There is another way, and if I do it again I might go this route. Instead of making a new top-out spring, just buy some new ones from Honda. They still carry them and they cost about $3 each. Since they are 1" long, adding two more to each fork leg will drop it about 2". Probably less, though, because they get compressed some when you put the main springs in. If you don't cut the main spring, the top-out springs get compressed more (making for less drop) because there is more main spring pushing on them. You can experiment by adding and subtracting springs to get the drop you want.
The springs are part #51412-340-000. On the fiche for the '78 750F forks they are #5.
Good luck,
~john
