Thanks to everyone for your help so far. This is my basic design. Overall footprint will be 63" x 48". Please take a look at what I've done so far and offer comments and potential problems.
1/2"-10 ACME lead screws
20mm Glacern linear bearings on X and Y
SRS25M THK linear bearings on Z
8020 sizes:
1515 and 1530
Z mounting plates 1/2" thick 6061 Aluminum
I look forward to seeing what you think.