Depends on how good you are. For a typical VMX rider, if you take any of the motors you've mentioned and get it running right, the motor will not be the limiting factor.
With few exceptions, its very rare for peak HP to be a significant issue in the results of club MX - its about who is comfortable winding the throttle on earliest, and keeping it there - so a good useable spread of power and properly sorted suspension are MUCH more important.
Personally, I'd go for the IT465 donk, although the YZ400 motor will probably be a good thing too. Side exhaust ports are a bit of a pain in most modern frames, though (if that's what you're planning).
The YZ360 motor is just too old and too valuable to be a good choice, IMO.
At HBBB, there was a mid/late-90s(?) RM with an RM or PE400 motor in it. I didn't check it out properly, but it seemed to be quite well done, including a custom alloy fuel tank.
All of that said, have a long hard think about what you're hoping to achieve. I'd had this idea of building an 03/04 YZ250 with a YZ250G motor in it, and some TTR or YZ-U cartridge RWU forks on it. I'd give it the YZ-G colour scheme (black frame, seat and fork boots, white plastics inc the old style guards).
Then I realised that even if someone gave me a perfectly good 04 YZ roller, the project would still cost me considerably more money than just buying a decent 04 YZ250... and that by any logical measure, the stock YZ would be a better bike than my hybrid.
Instead, I went and bought a good '06 YZ125 and spent even less money...
[/wet blanket]
