No worries welding two different thicknesses of metal...they wont be much different anyways. The 3 sided units are probably comparable to a 4 sided tube, but you will pay for them obviously. I REALLY like the idea of going through the Tbar crossmember and tieing them back into the frame rails. I have been thinking about building some as a production product...maybe I can make an option that goes through the crossmember.
By the way, where in Granite Bay are you? Small town...we cant be too far away. LOL.