Welcome David, it is plain that you want to jump in and start fabrication on your own project. I have no experience with the mfgr that you mention, i just assume i'll be basically unhappy with the product that they send and i have rarely been disappointed. The 'pay for what you get' analogy isn't true in our hobby - it is up to you to make the product 'special'. The guys giving you advice can take the same 'fixins' as you are contemplating and turn it into a beautiful thing while dealing with imperfections from the mfgr - and they would expect it, no big deal. What you do with it is the unknown.
I don't work with aftermarket modern tin, i work with the old original sheetmetal and i can tell you - without reservation - that the stuff they made (up into the '30s anyway), was rough, ill fitting and crude compared to todays standards. A truck i am working on now has a 3/8s difference from from door to door. I expect any modern mfgr of bodies will be more exact than that.
Good luck and keep us posted on how/what you do ok? oj