I did something similar back in the late 70's. I took a neglected Marshall 4x12, cut out the baffle and installed a new one with a single 15" speaker.
2 things I later took from this; what the hell was I thinking cutting up a 1960's Marshall cab and who the hell gave me it?
I do recall it sounded monstrous. Thiele parameters etc didn't exist back then ( ) but I'm sure the box size exceeded the drivers needs.