Without knowing the size and type of shafting you have, it would be hard to predict how it will act as far as deflection goes.
I used to design industrial machinery such as this, before I became disabled, and for the size of project you're going to do, shaft deflection will be a problem, unless you have some pretty big shafting and bearings. Shaft deflection can mean an uneven cut, and vibration.
Normally, for a project of this size, the shafting to be used on the longest length you have, which will be 8 feet, plus some for the carriage, should be the supported rail-shafting mounted down to a fixed and very sturdy table. This is the shafting that mounts down onto a solid aluminum riser, and goes completely under the shafting making it into a rail. If you tried to use, say two pieces of shafting only supported by its ends at around 9 to 10 feet long, it could cause a vibration when the cutter was mid-way in the work, especially if you took a very heavy cut. The other direction, or axis, will be a little over 4 feet, and you could probably get by with end supported shafting here, as long as its big enough.
An idea would be to take a look at a CNC routers construction of something this size, and see what was required to do it without vibration.
Last edited by WillMatney; 03-07-2012 at 01:31 PM.