I'm curious about your dimensions: 1200mm is three-quarters of an inch short of 4 feet,and 2700mm is 8foot 10.3 inches... You're really close to a 4-by-8 foot sheet, so that you could cut out directly from a full sheet of plywood. The Y-axis is probably 8 inches longer than you need, (you need some space at the edges for clamps, I'm guessing that 2 inches is probably enough), but the X-axis is too short by about 70mm. Seems a shame to come so close but not "do it right".

I can't claim to have a lot of experience with extrusion, but I would guess that you might have some rigidity problems with an X axis that is that much longer than the standard OX, but not beefed up in any significant way. Also, if you read http://www.openbuilds.com/builds/area-51-cnc-parts-f-117-cnc-gear-rack-drive-system.1428/ in full (yes, its a long read!) he started with belts and moved to rack and pinion because belts that length had too much stretch and gave him rigidity issues. As a result, I'd consider 2 things: 1) rack and pinion like what hytech2K did. I think he sells parts now, or you can look at cncrouterparts.com. 2) I'd look at c-beam with threaded rod. The longest rod that openbuilds sells is 1000mm, but I'd expect you can get longer somewhere. However, you might need to go with heavier rod than 8mm because it will flex too, and there might not be enough room with the standard c-beam hardware to fit a 10mm or larger acme rod...

 

... and I think I skimmed your description too fast. Rack&pinion on the Y axis is good! however, I still think everything I said is pretty much true...

 

Hey Mark! That's cool, but you're not selling acme rod to match!