Servo piston size and two very very minor tweaks in shift valve train. But not even noticable. That trans is a good one to build. Billet servo and you have every bit of a GN trans.
Two other minor differences are the speedometer gear on the governor (3.73 vs. 3.42 rears) and lack of a 3rd gear pressure switch on the Monte unit. Neither are a big deal to take care of.