I understand where you are going with this, but I think you're going about it the wrong way. The simple answer is, no. None of the suspension components off of the YFZ will fit on the Banshee frame without modification, especially the rear suspension. The 450 rear swingarm uses a completely different style pivot tube, and if I'm not mistaken, the chain alignment is about 5/8" off.
Simply replacing your rear swingarm, and shock with parts off of an 05 YFZ isn't really "updating" anything. It's more of putting different shaped parts on there, doing the exact same thing. The technology used in the YFZ450 shock is identical to the technology used in the first shock used on the banshee in 1987. Nothing has changed. Not the seal head design, piston design, valving design, etc. Nothing.
I've pretty much made my argument against using yfz450 suspension on Banshees, so I won't repeat myself, other than to say it's not a great idea, and I have seen very few actually work. The reason I'm telling you to not even attempt, is because this has been tried, and failed at, thousands of times. I get phone calls every day from people trying to make the stuff work, and they end up just throwing it away and buying the proper stuff. IF you get it to work, it will still be substantially sub-par to even the bottom of the line aftermarket equipment available for the Banshee.
Again, I cannot stress enough: Don't cobble together a bike. I personally don't feel that it is safe, nor cost effective in the long run. None of the components are "bolt on", and everything will require modification to some degree.