Hi... I never owned RB-67 or RZ-67 and didn't know a 6X8 back existed at all... But I did ask the most reliable source one could possibly ask as to have a reliable answer on Mamyia equipment... (His grandpa was a Mamyia dealer for 60 years and the dealership passed to him and lasted until P1 took control)...
Here is his answer: The 6x8 (power) back couldn't be used on RB-67 directly either... it needed the light baffle to be replaced with a special one (which was wider) as to accept the 6X8 back on it... Now this light baffle won't fit on the RZ at all as the body is a bit narrower... So... sorry guys, it won't work...