How about this Z axis?
I found this Z axis forsale at cnclinx.com. This one comes with a router and a stepper motor but I was wondering if I got this can I modify the router bracket to fit a porter cable trimmer router? This one on here looks more like a dremel size.
And to build a gantry around this, I was thinking of mounting some linear bearings on the back and use 2 linear shafts to support it for the Y axis and move it with the same size screw.
You will have to know the diam. of the larger router. Looks like you'll have to cut out more of that bottom mounting plate.(In the top photo)
Looks like the actual router holder is bolted on. I would unbolt that, then build a new bracket, for your larger router(save the original mounting bracket in case you want to use it later on)
Looks like a solid setup.