Bench Seat Replaced with buckets. Brackets/supports needed?

Weld up the cracks first. Prep the area, scrape the undercoat from the bottom, then wire brush clean, - grind down the paint to bare metal. Have a hammer and dolly ready. Immediately after you weld, hammer the crap out of the weld to stress-relieve it. Grind the surface flush to allow placement of the reinforcement plates. If it's all done well, the cracks won't return.