What is this bracket called

I agree. Not sure what kind of power plant you intend to go with but I believe that "splash shield/support would be tied together with the rocker, torsion crossmember, and frame rail on a 4spd car from the factory.(tourque box) The factory also installed rear torque boxes just in front of the rear springs. (Again on a 4spd) My opinion is to box em in. Don't remove. You may disagree.
The Engine will be a 16 plug gen 2 hemi prostock motor. Like I said prior I dot intend to eliminate it I will instead remove it and run a 1 5/8" chromoly tube where the panel was as I can't find a replacement. The car is a coil over car and doesn't have the torsion bars so so the need for boxes are none. the rear frame rails are goin to be split in half moved in like the butch leal A/MP car with the leaf spring boxes moved to match the leading edge of the 1" wide rail