I have a '74 Square C20. It had an OEM 350/350 combination and a TransGo kit when I bought it. It runs 3:73 gears. I drive mostly highway.
The OEM 350 was replaced with a pro built 355 SBC, mild cam, Heddman Headers, Edelbrock 650, and Edelbrock Air Gap manifold. The TH350 was replaced with a performance 700r4, mild stall converter (around 2000), and no lockup. Final drive is around 2:60.
The OEM wheels and tires were replaced with a new combo that was +/- 1/2" - coming in around 30.5-31 inches.
The OEM gauges were replaced with Speedhut gauges. The fuel gauge has been calibrated and verified accurate. The electronic speedometer has been calibrated with GPS and has been verified for accuracy.
In OEM form, I was getting between 12-14 MPG. With the Edelbrock 650, Air Gap and 700r4, I dropped to 7-10 MPG.
I was expecting at least 14-16 MPG, not the drastic decrease. I read where people say they are getting 16-18-20+ MPG even with BBC's. So, my experience is not anything close to expectations or claims from others.
We have reworked the timing several times, gone through the new 650 three times, there is no explanation. I have been told that the Air Gap is not allowing the fuel to atomize from the hot oil in the valley, like a non-Air Gap. . . among other interesting things.
At this point, I am debating whether an EFI kit from FiTech, or someone else, would change things for the better, but want to know what the issue is before dropping another grand.
All this to say. . . You can't always rely on the OD Transmissions to deliver the results you expect or what others say they are getting. Sometimes you find yourself chasing numbers and bleeding cash.