And you can use later model dust covers to keep the crap outta your seals, too.
Try this: With the bike on the centre stand (if you still have one, otherwise use a helper) stand at the front of your bike, with the front wheel clamped between your legs.
Try to twist the handlebars from side to side, and you may well be surprised at how much movement you have without a steel front fender or fork brace, or better still, both. This "flex" translates to crap handling in the twisties.
I seriously doubt that a fibreglass fender held on by hose clamps or zip ties will add very much to the strength of your front forks. On our old bikes with long skinny fork legs, the addition of a decent brace will really assist in tying the legs together and making the whole assembly considerably more rigid.
I did the "twist test" comparison on my semi-restored K3 with no front fender (as yet) and my K1 with new CycleX two inch over fork tubes, later model K8 sliders, with a German "Telefix" fork brace, and the difference was night and day.
Once I get time to scratch my asss I'm going to machine up some more fork braces like the one I made for Sean Condon's F2, and the first one will be for Derek. (seaweb11)
It's almost impossible to make the alloy braces fit the earlier CB750's, but you can easily fit the later CB750F/F1 or K7/K8 sliders to the K3-K6 forks, or the whole front end assembly to the K0-K2's, the sliders are an inch or so longer and allow the fitment of a decent brace.
There were some spring steel "loop" type braces around back in the day for early CB750's, but they're pretty hard to find nowadays.
Once I get home I'll reduce the size of the pic of my K1 so I can post it here, it's running the 2 inch over fork tubes, K8 sliders and Telefix brace, and due to the longer sliders and brace I can still use my OEM fork "gaiters", so it still looks kinda OEM. Cheers, Terry.