I am thinking that if you are careful, do your research, and purchase a disc/drum master cylinder with a smaller size piston, you will be sucessful. I assume you are doing a fire wall mount where the physical size of the booster will not present a problem (under floor setups can get tight). I would not go less than 8 inches. You may want to look for a donor car/truck to cut your cost. Please remember, you have to have good brakes! Get help from someone who can lay eyes upon it if you are unsure of what will work. Good luck.
