Oops. 2 1/2 or 3 inch exhaust. Would either of those help much on its own for now?
Also, a guy on a 351 C forum suggested that the wire from the 50 year old ignition switch may not be providing a full 12 volts to the coil. Could this be a likely problem? He said maybe bypass it directly to the battery with a relay to make sure 12 volts is delivered.
Would a MSD 6AL also help this engine much with performance?
As for increasing pipe size, it will definitely help. Some of those 351C exhaust manifolds aren't that bad. Way better than what comes on an Windsor! lol As for mufflers, units like the Walker Dynomax are 'see through' cost zero horsepower, yet are still very effective. They work through sound ABSORPTION. The longer they are, the quieter. With a larger car, there is usually enough room for the longer mufflers, and you'd be surprised at just how quiet they can be.
As for pipe size, if you're using manifolds, you're probably limited to whatever size fits the manifolds. Of course, you could always 'up' the size using an adapter. 2.5" is what would probably work the best. Are you using tail pipes? If so, research what MANDREL BENT tails will fit your car. Flowmaster makes tails for less than a shop will probably crush bend a set for. With all the bends of a tail pipe, you're ahead to go this route.
BTW, what kind of car is it?
MSD boxes are nice, but in my opinion, you won't have a decent baseline until you get the exhaust freed up... as you will NOT do the engine justice until it can breath...