That's an impressive build, Kawfi! The steps you went through in flattening it are ones most people skip. Since you were making epoxy lands for your rails, do you think it was really necessary to flatten the steel first? (I'm hoping you'll say no, since I'm contemplating a similar issue in an upcoming build and hoping to get away with epoxy alone.) Your expectations sound pretty reasonable, considering how heavy-duty you're making this thing. What are you using for a gantry beam, more of the same 3"x6" tubing with more plate stuck on,and more epoxy on top of that? How much Z travel will it have, and what kind of spindle do you plan to use?
As long as it's all in your head, I'd say get it down in CAD. If nothing else, it will be good practice, and it could help you clarify issues that haven't come up yet. Once you've got the machine up and running, you'll need those CAD skills to design all the parts you'll be able to produce.