[Okiesnake]

I also have a 351W with a MSD 6AL box and a Ford Duraspark distributor.
First of all, don't hook up any of the Painless wiring harness to the coil. The Painless wiring harness assumes you are not installing a high output electronic ignition. All that you should have hooked up to the coil is the thinner black (-) wire from the MSD and the orange (+) wire from the MSD.
The Heavy Red (+) wire from the MSD to the battery side of the starter solenoid and the Heavy Black wire (-) from the MSD to a good frame ground.
The black/violet wire from the MSD is butt spliced into the green wire of the distributor and the black/green wire of the MSD is butt spliced into the violet wire of the distributor. The black wire on the distributor is not used.
The thin red wire from the MSD is attached to a switched 12 V source (i.e. the ignition key switch).
The white wire from the MSD is not used.
Take an extra piece of wire and put an electrical female spade connector at the one end and attach it to the male spade connector of the MSD (Tach Output connector - the little black round connection on the end of the MSD). The other end of this wire is attached to the tach (S) signal connector on the back of your tach gauge.
Your harness probably came with a fusible link. You need to install it where they instruct (at the battery side of the starter solenoid).
Good luck, hope this helps, been there - done that, had the same problem myself.
Okiesnake