I got 21 mpg hwy regularly in my 69 slant. Haven't driven my 64 slant much yet. I stopped fooling with points long ago and convert all my cars to electronic ignition. Cost ~$20 to put HEI in a slant once you find an e-distributor (73+? cars). Run the most spark advance you can without pinging going hard up hills.
When the exhaust was off did you check that the "heat riser" is not stuck closed? You don't need it in CA and can block off the shaft holes. Stuck open is OK too. To test for an exhaust restriction, put a vacuum gage on the intake. Crack the throttle. The pressure should instantly rise (less vacuum). If you see a slow increase in pressure after that (>2 sec), that indicates an exhaust restriction.
The carb would be my next thought. Next time you have the exhaust pipe off, cut a hole and weld a bung for an O2 sensor. I put 2 bungs in to allow a wideband sensor and regular (to calibrate first). Without a sensor, you are flying blind. The only alternative is to inspect the spark plugs and guess "lean or rich". Try leaning the idle mixture since probably rich. Optimum for mileage is just before you get lean mis-fire. To lean it above idle, you must decrease the internal bleeds. Some people do that by coating the holes in their existing ones w/ solder.
Finally, check compression. A nice test kit is $15 on sale at H.F. Without one, remove the oil filler cap and the PCV valve, rev the engine, and note the blow-by gases coming out the valve cover that went past the piston rings. A lot of smoke is bad. Noticeable puffing might be one bad cylinder. You can also turn over the engine by hand (pull & push belt) and feel for 3 good cylinders compressing per rev.
. You say you got it back on the road. If it set for quite a while you might check the brake system for any drag that could've developed. Tires inflated enough? Timing/advance as stated above? I agree 13 mpg with some highway included is not good.
