If your 100% sure your gonna build your own from scratch...Go ahead and buy the SFI spec manual that tells you step by step where bar placement goes along with proper wall thickness and OD on tubing...don't know what type chassis you have u have in mind but you can get the SFI SPEC from 6.0 moly on up...of course Alston, chasssis engineering are two very reputable vendors on selling un welded kits.
If it were me I would check into the pre fabricated cage and build from there..at least you know you are gonna be all over certification and still plenty of fab work left for you...and the price is not much difference to me anyhow for the time etccc you will have invested at this point.
JMO
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "