My understanding (obviously not first hand) is the plastic tank sits lower in the chassis and thus decreases road clearance and thus exposing it to potential damage. Unknown whether the SC undercover will fit with this tank, but the SC cover only extends up to a factory tank (see photo).
A friend with a '73 removed the plastic tank due to it hanging too low (bent a supplied cover attachment which then kinked a fuel line). The front ride height is a variable.
The aluminum crossmember is a direct replacement for the steel version. Haven't heard of any differences. Search the internet for pics to compare and confirm.
Here's an SC tank in my '69 with SC front suspension with its al. crossmember.
Before folks get upset about my floor jack placement, I have installed a long ~8mm spacer to span the normal space between the cover and the crossmember. This results in a handy front/central lifting point w/o bending anything.
Sherwood