noob needs help with mileage - only 10 MPG!

If it runs rough and has excessive leak down readings (#1 35%!), you have mechanical issues. Valves are probably toast, could be rings. The shop should have told you where the air was leaking from. A new carburetor and a tune up ain't gonna fix your problems. General spec for a healthy engine is 10% between cylinders. Like said above, anything over 20% leakage usually warrants a tear down.

Before leaping to any conclusions though, perhaps you could try some combustion chamber cleaner treatment type thing to see if that might free things up a little. Could be a lot of carbon in there causing problems. Mopar makes good stuff available at the parts counter. I think it's like $7-$8 a can. Worth a shot. Least it will be cleaner when you take it apart.

I was going to ask if you had an electronic ignition and it looks like you took care of that with the Pertronix. However, I hope you/they looked carefully at the distributor before putting it back in and verified the rest of the components were good. Might be excessive play in the shaft which could cause your timing to be erratic/unstable. Slants have a plastic gear that can get worn/chipped. Just a thought.

BTW, new electronic ignition packages are relatively cheap and easy to install. I put one in my old '65 Valiant (Slant) and it was like a new car. Seriously. Better mileage and all around better performance - start up, idle accel. Great bang for the buck.