I recently purchased a 1977 Ford Diamond Mini-motor home with intentions of refurbishing it and hitting the road. The running gear is basically a stock Ford E350 Econoline. It has 26,000 original miles. The unit weighs around 9000 lbs, loaded. Some preliminary research tells me that the transmission is a C6. The 460 does not appear to have any modifications. Any engine upgrades that I will do will be based around gas mileage and reliability. A performance increase would be nice, too. My planned engine budget is $2000.
All work that I do will have to be done with the engine in the vehicle, since I don't have the equipment or facility to pull it.
Within my budget, what modifications make the most sense for this project based on best possible gas mileage and reliability?
Since it only has 26,000 miles on it, all should be good with the engine unless something foolish was done to it. You could do a compression check to verify.
Be sure that you have adequate cooling for the transmission's fluid. Add more if it's insufficient.
If it still has the factory's cam gear in it change it to an all metal gear. That also would be a good time to reset the cam timing to pre-smog.
Many RV owners have realized performance and fuel economy by improving exhaust and cold air intakes. Here's one source:
Banks Power | Diesel Performance and Gas Performance Products
Your ignition will likely be a Duraspark II. If you want more spark switch to a Duraspark I or make your own HEI using a GM module and coil. Relocate the module to a cool location and always carry a spare for any semiconductor controlled ignition.
If it has old tires on it (over 7 years) replace them with new radials. That will likely have a great impact on your budget. Always keep them inflated properly.
Try to get it to run as cool as possible. Heat has ruined many engines and transmissions in RVs.
Be aware of the emission testing requirements for the area where the vehicle will be registered. Some smog equipment may be able to be modified to get better fuel economy but if it can't satisfy the law it's not worth it.
RVs are not famous for great fuel economy. Drive in a conservative manner.
Check the brakes thoroughly. Brakes that drag will kill performance and fuel economy.
Always keep everything in a good state of tune and maintain fluids and filters properly.
Use engine oil with sufficient zinc compounds added and viscosity for your environment. This will discourage wear & cam failure.
Grease the chassis regularly including the U-joints. If it's got a two piece driveshaft check the center support.
Get a set of Ford truck shop manuals for the chassis you have. Read it, understand it, and follow it. Originals or CD reproductions should work.
Be sure that either the carburetor's automatic choke works correctly, or switch to a manual choke & throttle set up. It should close on a cold start and open completely after warm up.
Don't run on any higher octane fuel than is needed to get the engine to run properly without knocking.