Years ago I installed an MSD 6 to my '75 2F. The only real benefit I experienced was a cleaner plug, less fouling and therefore longer plug life. No noticable improvements in mileage or power. I recently switched from the stock points distributor to an electronic system from a '79 cruiser. This made a considerable improvement in power, idle, fuel economy and smog readings.
If I were to do it again, I'd convert to an electronic system ('79+ toy, HEI, DUI, etc.) long before adding an MSD.
BTW - the offroad version of the MSD is just more water resistant and vibration resistant than the original unit. This is something that can be accomplished with a little silicone sealant and some rubber washers IMHO. I've been running my standard MSD6 this way or over 9 years without any problems.