newbie needs help with seat install

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hey all you pros, got a question for ya. I am installing some new bucket seats in my 70 nova and am having hard time deciding how to install them properly and safely. my problem is the car didn't come with buckets seats from factory so I only have two mounts per side on outside. my planted seat brackets are meant to be bolted in place of stock buckets, im missing the inside brackets. I know I could go buy stock brackets and weld them in, I guess, but.......so my question is this: if I make my own brackets what is the safest and best way to mount, welding in brackets to floor pan? if so what thickness do you use for brackets? drilling holes in floorboard with plates underneath? I don't know, what do you guys do? I want to be safe and secure as this will be an autocross car.