[rich weyand]

I'll second most of the other suggestions, and add a bit:

A wideband A/FR meter works wonders in getting it right. I did all the tuning steps, then bought the meter and found out I was still way off.

Make sure your vacuum advance is on manifold, not ported, vacuum. Ported vacuum will cost you mileage. At idle, there will be vacuum on a manifold vacuum port, but not on a ported vacuum port.

I am running a 78 K10 with 350, TH350, 3.73:1, a modded NP203 with locking hubs, and 31/10.5R15 tires. I get 9 mpg short trips in the city during the winter, and 10 mpg short trips in the city during the summer. The difference is the choke being on and running a richer mixture until the engine warms up. On the highway, I can get 14 mpg at 55, then it starts going down as I go faster. That's all hubs FREE; hubs in LOCK is about 1 mpg less.