I've had an MSD6a in my '85 GT for 3 or 4 years without any problems. It works great, although you'll also need they're tach adapter for the factory tach to work. If I had it to do again, I would go with the 6al box with the built in rev limiter though.
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'85 GT / '72 Mach1
Long list...check website!!
members.xoom.com/Bobstail