So a quick disclaimer, anything I state is merely a subjective opinion and should not be relied upon for anything other than entertainment purposes. Use of such opinons for any other purpose could result in serious death or injury. Sorry about the ridiculous disclaimer but the world is what the world is.
I modelled up a crude representation of the KX450f frame with the Y moved up in the configuration I used for my conversion to the KX500 engine.
Based on this model and a very simplistic analysis I've reached the OPINION, which I refer you once again to my disclaimer regarding the appropriate use of, that thin wall square tubing is woefully inadequate even with a very rigid head stay in the region directly below the Y.
I'm unable to submit recommendations due to liabilities so I'll leave you to your own conclusions.