#1
Would i want to put a board up over the back?

Would being closed sound better?

Would i want to consider a different speaker for a marshall 18 watt clone?
Peavey Limited Exp Hss
BC Rich BW-4000BB
Schecter C-1 Classic
#2
I'd just try it like clamp a board to the back and then take it off and compare it. That's what I'm going to do with the cabs I'm making except I'm making some removable backs for it so I'll have the choice if I ever want to change it.
#3
dont close the back unless it was closed before. closed back cabinets require alot of measurements, air space dimensions, etc.

open back is better all around, although it doesnt matter so much with GUITAR amps, bass amps on the other hand are another story.