I think your best bet is to just go ahead and put the shifters in place where you want them,mock everything up until its where you are comfortable with them.
Then start worrying about the console and design around everything..1"x1/8" square tube should be plenty.
16 gauge will be good also,but remeber the lighter the gauge ,the harder it is to weld without blowing holes and a bunch of warpage,so it can depend on the experince you have in doing it.
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