Sure glad you're in one piece, Jaknight! That's one tough ride...
The stock springs were good rates, but sagged early. After that, they were pretty stable. Most of the time, cutting a piece of 1/2" electrical conduit, to 1/4" longer than the unloaded slack distance in the tubes, works real well for perking up the CB750 front end.
Put the bike up on the c'stand (you'll probably need a little help, right?), then lift the front off the ground by jacking up the engine or frame. Remove the fork plugs and you'll probably see a "slack distance" between the end of the tube's threads and the top of the springs. This is the "sag distance". Make your spacer either equal to this distance (normal riding) or add up to 1/4" extra length to create more tension (good for 2-up riding, canyon racing, heavy touring).
If you want progressive action, add air forks. This is as simple as drilling and tapping the fork plugs and installing Schraeder valves. Run 5-15 PSI, experimenting to your taste. This is the ultimate in tuning for these bikes' front ends.
