Howdy emullen,
I chose the 450f frame because it appeared to me that the engine cradle and the down tube on the 250f frame were prohibitive. The engine cradle tubes begin an upward bend too early to meet the down tube. The down tube appears to be at an angle that would pass through about where the front of the 500 cylinder needs to be. That does not mean you couldn't cut out and weld on more of the down tube (starting closer to the steering head tube) and cradle, but the new piece (down tube) would be welded on at a different angle than the original. This in my mind puts up a lot of red flags, the load that passes down that tube needs to continue along the same (original) plane. Also there is a second wishbone that attatches the down tube to the upper frame spars. I would not recommend cutting above that point. That actually carries more load than the engine cradle/wishbone below it. Lastly, the cast piece on the 250f where the swingarm bolt passes through has a different bend to it. I think this might interfere with a 500 kickstarter.
Keep in mind I have never actually tried to set the 500 motor in the 250f, all of my conclusions were based on photos and simple overlaying a 500 motor on the two frames in photoshop. Hit me with as many questions as you want, I'll be glad to help.