The rubber brake hose is usually the culprit because they collapse on the inside and you can't tell if they are flowing, no fluid out of your left hose pretty well tells the story. New brake hose, bleed and you should be all set.
Don't feel bad, I get 6 to 7 around town but that's all I expect with what I'm running : 468 ci,big solid lifter cam TH400, 4:56 gears, 31x16x15 tires.
In my 76 w/ 468 I run an aluminum 4 row radiator w/ stock clutch fan (7blade bb ) and temp stays @ 180-190 w/air on in 90 degree heat. You also want to run a shroud.
You guys should join the Antique Truck Club of America (ATCA ) based in PA. They have chapters in many states and include all types of trucks.
More info @:Antiquetruckclubofamerica.org
I have tried the Holley pumps and they don't put out the volume or pressure. I wanted a pump I could use without a regulator.
Back in my racing days I used 2 Carter rotary pumps in parallel, worked great. I now run 1 Carter pump w/ external pressure regulator set for 6 lbs pressure. If you are...