http://www.lansingheritage.org/html/jbl/specs/pro-speakers/1984-4430-35.htm

The 4430 uses the 2344 horn so it's impossible to copy JBL's design on this one.

No problem in changing cabinet dimensions so long as internal volume is kept the same. There are changes - a larger baffle means that the frequency where it behaves as though its in half space condition is lowered, and vice versa; smaller baffles raise the frequency where the baffle forms a half space condition. Sound becomes more constrained with a larger baffle, i.e: increased directionality. Changes of baffle dimensions and shape also affects the severity of cabinet diffraction.

About the changes you want to do however - from 31*24*23" to 36*20*21", is a small enough change that you don't need to worry about anything.

Adrian