Making a custom truck bed frame

[Dodge68charger]

Hey guys new to the whole truck scene. I have a 70 chevrolet cab and im trying to slowly collect parts for it. I want to go the lowrider route with huge wheels and tires. I ve been doing some research but not sure how to go about building a steel frame truck bed support setup. What size tubing, what do i weld to, do i weld it to the outside sheetmetal, etc. any help would be great. Thank you
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