Washers (no more than two ea) between driver side studs of the booster spacer and the firewall will usually angle the booster justvenough to clear the shock tower. 

You will need more than two washers per booster stud because of the hollow back of the booster plate    Add enough to just barely angle the booster.

Normally, once the booster and mc are completely bolted up tight, the mc will just barely clear the shock tower.  Of course, no two Mustangs are the same. 

I do have an extra Midland-Ross booster that has been rebuilt and restored that I bought for my Shelby. Since I sold the car, I will be selling the booster.  If you have tried to find one lately, you know they are getting hard to come by.   It MIGHT give you 1/8” extra clearance.  But it is not cheap.  I actually prefer the Bendix version for performance.

Should you decide to grind the end of the mc, take the pistons out and measure to see how thick the end is before you modify it.  I would think you would want at least 1/8” thickness left after grinding for clearance.

50vert wrote:

To get around the hollow booster plate, I loctite a nut on the studs,at the right height, then add a washer. It gives something solid to tighten against.
Set the height of the nut and washer to be level with the booster plate. If you need to angle it, then put an extra washer or two on the driver side studs.

 
I used to just stack washers on the studs to take up the hollow space, but I like Barry’s solution better!