I didnt have to cut the piece behind the seat off because my seats were mounted above it, but it does look like some type of cab support.
When I put "aftermarket" seats (other than stock) in my '53 chevy, I made the brackets from electrical conduit strut . It's called kindorff strut. It comes in all sizes like 3/4"x1 1/2" x 8'
all the way up to 2"x2"x8' . it comes solid or with holes pre-drilled the whole length. It welds great and you can also use 3/8"or 1/2" all thread(6'threaded rod) top and bottom the rod with big washers and nuts,tighten your seat down and put it where you want. This way if your seat and strut is far from your floorboard the all thread can be cut off where you want instead of trying to find 2' long bolts.You can also buy spring nuts to fit this strut in case you want to weld your strut to the floor and bolt the seats to the strut. Go to any electrical store and they can show the types of kindorff you can use.
Hope this helps,
Good luck
