If you intend to drive the car on the street, even with some race track time, you need vacuum advance. Much better gas mileage and drivability.
Unless you want to spend money unnecessarily, I suggest getting a Ford DurasparkII distributor (1985 specifically if you have roller lifters). Then get the MSD 5 or 6 series control box. This combo will work as well as a MSD distributor and will save you much $$.
This purely my experiance. Yours may differ.
I had a vacum dis on my 351C and got 12mpg on the highway with a 625cfm Road Demon carb. I have a MSD pro billet mechanical dis on my 429 with the lightest springs installed. I get almost 11 mpg on the highway with the 750 cfm speed demon. . But I do agree if it is just a run around car and ease of use get a vac distributer
I have the MSD Billet #8582 (mechanical w/ lightest springs installed), 6A control box, and Blaster SS coil installed in my stock 302 (stock longblock anyway). Most of my driving is from home to work and back, which is about 3 miles one way. With a speed limit of 30mph here on-base the car barely warms up to operating temp before I have to shut it off. Anyway with that said, I average about 14mpg. As far as driveability, once the car starts to warm up, it drives great (it doesn't like the cold here in WA, neither do I for that matter). Engine Compartment
Several years back I wanted a MSD setup for my 351W cougar. I waited for almost a year for those MSD people in El Paso to make up their mind on giving us FORD guys a vac. adv. distributor. They decided not to at that time so I finally ordered a 351W distributor the only way MSD made them back then...Mechaical advance. I still would not mind a vac. adv. distributor for the daily driver cougar but I am not going to fork out the cash to change out the perfectly good mech. MSD in it. On Highway trips I can get 21 MPG with it at 70 MPH. The Vac. assist distributor they make now would be nice to have for street driven cars. John