There actually is a good bit on modifying F/X frames in the rule book...
Big question would be is this a Mid engine "squared" chanel frame or a "Pan/Stamped" frame...
My understanding is the Chanel frames can pretty much be "replaced" as long as retain the "form" of the original factory frame... (but no round tube frames)
The "Pan Chassis" frames can have wheelbase altered, but must retain original "Pan" style chassis and body parts (tunnel, rear sub frame etc...)
So no "tube framing" the rear of a Pan chassis..if that's what your asking...BUT..you can discretely reinforce the rear axel mounting area...as long as it's covered by the original sheet steel...