Economy and heat riser valve on pass. exh. man. questions

[83GMCK2500]

I was reading one of the quadrajet tuning threads and I noticed that the vehicle in the thread had a vacuum operated valve down on the outlet of the pass. exh. manif. I have one with a counterweight and clockspring on my truck. Does one work better than the other?

Driving ~75% city/25% hwy I was at 10 MPG on 91 octane with my last tank of fuel. (On hills the truck pings with anything less.) Those are 25 & 35 zones in town and 55 & 60 zones on the hwy. Details not in my sig are 4.10 gears and 265/76-R16 tires, speedo is very nearly dead-on at 60mph.

Is 10 MPG reasonable? If I posted detailed pics of the topside of my engine can we verify that all of the vacuum lines are hooked up correctly? The truck runs very well right now, is just thirsty. After reading that other tuning thread I would like to verify that everything is in the right place.