Be aware that wherever you weld this to your hull better be super rigid or the hull flex will cause a crack at the weld joint. If you want uber rigid, run multiple smaller tubes stacked and plate it....you'll have to have bad luck and a big wrench to crush it.
I personally would avoid welding to the floor unless you can plate a section to ease the flex on the weld area. If the transom is rigid and well attached....its not going anywhere that a 20hp is going to hurt it.
I'd go 4043 on this one myself.
