Hi,
I know that by 1979, the largest gas engine in the Dodge lineup was a 360" V8, and that is considered one of the Dodge smaller blocks. I really don't think that the 6.7L diesel will fit, and even if it does, the engine you are taking out is about 400 pounds while the 6.7L will be about 800 - 1,000 pounds. Then there is the intercooler, larger radiator, and oil coolers. You also have to figure out how to connect the new electronic controlled engine and new transmission, as the older Dodge transmission will not hold up to any diesel's torque rating, and is only 3 speed, no overdrive.
So first thing you should do is check the weight of this motorhome. If it is single rear wheel, forget about it now, the chassis is not up to carry an additional 400 pounds. Many times Lazy Daze and others put out motorhomes with a higher curb weight than the GVWR of the chassis they where mounted on. That said, Lazy Daze is one of the best built class C motorhomes, and thus you can still find a roadworthy 42 year old RV.
I know that the early 70's that the larger engine was only a 343 engine, so I don't know for sure what will be in your RV. It might be a 440" engine, but something tells me it will be much smaller. If it has a 440" engine, then that engine is closer to 500 pounds, so you will not be adding to much weight to the RV.
How about looking for a early diesel pusher, or searching for "Diesel" in all the RV search pages, and look for something built with a diesel engine?
I know a guy who put a 460" engine in a 1964 Econoline pickup. It was quiet a project, first he had to located a 3/4 ton pickup chassis, then cut it down to size, install the cab over the engine, and then modify the back pickup bed to fit the chassis. He ended up cutting 22" out of the center of the chassis, and increased the length on the bed to 9 feet. He also had to build a pair of 35 gallon axillary fuel tanks for inside the bed, then uses the rear tank last, as it is the only one with a gauge on it. He is very talented, and did the whole build in his garage. Yet as "Warm Up" projects, he also did body work for years, and when his dad's 6.9L F-250 was crashed, he also replaced it's cab and did extensive work on it as well, including installing a turbocharger.
Good luck with your project, personally I think it will turn into a "Special" vehicle that you will not be able to sell, as it is modified by the owner and might not be considered stock or be easy to insure.
You might be better off buying a used E-450 shuttle bus with a diesel engine in it already, try for something 2003 and earlier with the 7.3L diesel, it is much more reliable than the later 6L or 6.4L diesels.
Also buying a 6.7L engine, are you ready to spend $15,000 on one? And then you will need a DEF tank, and all the emissions controls to go with it. Probably another $2,000 for the automatic transmission to go with it. It is an expensive ordeal. Unless you are able to pick up a used F-250 and are able to take everything off it, and transfer it to the old motorhome, and then sell the cab and chassis for a decent amount, you will be out a lot of money.
You might actually be better off looking around the junkyards for a used Cummings diesel. They are more available, and not quiet as heavy. You are still looking at buying a used crashed truck for about $5,000 - $8,000 and then transferring everything over to the RV. It is a project that will take about 6 months and probably work on it about 20 - 40 hours a week. If you hired it out, well might as well buy a new DP at the labor rates of $100 per hour that most shops charge.
Fred.
