MSD -vs- Jacobs

If you are only getting 10 MPG you have issues that no ignition system will solve. 16-18 MPG would be more like it. 20 on the highway isn't out of the question.

A 1407 is a lot of carb for that combo, and Edelbrock jetted them "for performance". DOUBLE CHECK YOUR FLOAT LEVELS!!! Make sure the metering rods are staying down at cruise, and go 1 step leaner on the primaries (carb manual has good graph). Make sure the vacuum advance is working and connected to the ported (passenger side) port on the carb. With stock compression I would suspect you'll do best with about 10-15 degrees advance at idle.

As for the 318 exhaust manifolds, they will be fine on the street, but will hurt top end (above 3500 or so). Unless you are still running the stock 318 single exhaust, I wouldn't worry too much.