Ok, here are the cold hard facts...
Fender cabs really took a nose dive in the 70's and that cab is no where near as solid and sounds no where as good as a BF era Fender cab. They have a non floating baffle board that is made out of MDF where the BF era cabs had a floating plywood baffle, also the cab itself is simply not as well made and used lesser quality wood than Fender had used in the past.
Fender switched the Super Revereb to that offset pattern in 1970 and ran it until 1981 when the Super Reverb was discontinued and the size of the cab DID change...BF era Super Reverb cabs measure 25.5 x 25.5 x 9.75 while the later cabs measure 24.5 x 25 x 10.
As to where to getr a working Super Reverb chassis...look all over the WWW, check Ebay, fortums and chat rooms...short of that contact a small builder and see what he can do for you...I know that Mike Clarke builds Fender amp clones, he doesn't offer a SUper Reverb but he might to a one off if you asked him to, also Marsh amps offeres several BF Fender amps, and SDG Vintage does too...in fact SDG Vintage offers a Super Reverb clone you'll just have to see if they will build and sell you just a chassis.
Good luck