Per NHRA rules moly MUST be tig welded so it does not really matter IF it can be welded with a mig welder for the purposes of this discussion. As for attachment of bars in a unibody car it is acceptable by rule to weld the bars to 6"x6" plates to the floor.
Both true. Specs and construction techniques in the rule book are understood to be minimum acceptable. Fabing a frame structure in the floor, and building the cage on that rather than the thin sheet metal, will greatly add to the strength of the car. Safer, more consistent and responsive to chassis tuning.
