Welcome to iRV2 .
I was working at a Dodge dealership in 81 and due to BIG financial issues the factory was doing some strange things . Don't get me started on the Class A chassis issues.
I believe that the MB300 would be correct for the F40 , but I'll have to see if I can confirm .
Do you have any info from the tags that gives axle weight ratings ?
EDIT: Didn't find exactly what I was after , but info from this site could be helpful .
Master cylinder location is the same , the difference would be the " bore " diameter of the cylinder . You may have to pull the cylinder and measure.