Fuel Economy- A Funny Thing Happened

[SGS]

This is going to be a very long post. Thats your warning.
None of the new diesels get the fuel mileage that the pre 02 non EGRs did. The power is awsome on all of them stock or modified. In addition to 02 emmission standards that we have to live with, the OEMs continue to boost horsepower and the fuel mileage continues to drop. I have been thinking for a long time of trying to build a pickup to see what could be obtained for mileage. I have built plenty of hot rods and race cars in my life but never a diesel.
I was on a trip from East Tenesee back to Wisconsin. I was pulling my 3 horse 12' living quarters trailer. I would estimate my total gross weight to be pretty close to 18,000#.Just North of Nashville the intake pressure hose blew. It was a Saturday night about 11:00 PM and the odds of finding a Banks intake hose was 0. I was tired and wanted to get home so I kept on driving. The truck made a lot of noise and smoke. It definately lost power. The funny thing was that as sick as the engine was I was able to maintain a pretty steady 60 mph, sometimes 65, less often 70, and I never once dropped below 55mph.
I was curious and had the truck dynoed when I got back, before replacing the hose. The sickly thing pulled a whopping 97hp max and surprisingly 284 pounds of torque.
So now back to the project of building a top fuel economy truck. Question, If class 8 trucks pull 80,000 lbs at highway speeds with 12 litre engines why do we need 6 or more litres to pull 20,000 lbs in a pickup? Remember need and want are two different things. Wouldn't it make sense that 4 litres could pull 20,000 pounds efficiently? If I could pretty much maintain 60mph with 97 hp and 284 lbs of torque would 180 or 200hp and 450 lbs torque do an addiquite job on little fuel. remember i'm not talking about racing up a mountain just a good efficiant driver.
With that in mind I purchased a 1974 F350 crew cab 4x4. I purchased a 4BT cummins engine to install in it. 4BTs in marine trim pull 250 hp and 525 lbs torque. I don't think I need to go that high. the first gen 5.9s were around 180 hp if I remember right. I am going to use 225x19.5 high pressure tires to eliminate the extra friction of duals. Four 225x 19.5s with forged aluminum wheels also have a lower rolling weight than six 2.35x17s. A six speed .73 overdrive trans will hook up to ether a 3.54 or 3.73 ratio rear end. I plan on using an electric fan and fuel pump to reduce parasitic loss. I may even go back to manual steering.{remember I'm old and had plenty of the old armstrong steering trucks} AC, no way. I am going to use a Piers Diesel high torque cam and will probably port and polish the heads. We will add a frontal cooler and try to find a turbo that spools fast but not exsessively high. 3" exhaust should be plenty for four litres. Our local injection shop recomended six hole injectors for a more complete burn and they want to play with the timing while on the dyno. We will use a large radiator and I think a reletively high temp thermostat will make it burn more efficiently. Synthetic lube throughout will be used. I think the 74 in single rear wheel trim will weigh in about 2,500 pounds less than my 06 dually.
My goal is 16mpg or more when towing total gross weight 16,000lbs. 60 or 65 mph works for me.
OK Gearheads, what am I missing?