MG Mueller,
When you bolted on the cb500 front end, did you swap out the entire triple tree/fork assembly, and simply bolt it to the steering head?
Unless the fork size data provided at ohiocaferacers.com is wrong, the cb350 twin uses 33mm fork tubes, while the cb500 would be 35mm. That means that you could not simply swap forks from a cb500 into the triple tree from a cb350 twin. (Or if you did, the clamps may not hold the smaller forks firmly.)
Lokai, the 450 has the same 35mm diameter forks as the 750, so if you wanted to simply swap the 750 forks into your 450 triple tree, you shouldn't have any problem. The 750 forks may be a bit longer, but you could just slide the extra length up in the triple tree.