I have found that the body side moldings are usually the same as the Cadillac Coupe DeVille. The rear quarter molding is the same, and the same goes for some of the door body moldings. You will need to measure the length, and then start comparing them to previous year cars to determine exactly what they used. On my 1978 Superior, the front door used the same molding as was used on the 1978 Cadillac 4 door, and the rear quarter molding is the same length as the Cadillac coupe quarter molding. If you get stuck needing a piece of side trim, you can always have a longer piece shortened, or have two shorter pieces welded together to make the length that you need. On my 1978, the rocker molding is from the 1978 Cadillac limousine. Things such as glass are proprietary on the doors and quarters for each body builder. The windshields are all the same, and are still available. Don't just assume that a part from a regular Cadillac will work on the commercial chassis, since many of them are specific to the commercial chassis, but look very similar to there counterpart on the regular production car. An example of this is door handles, some brake and suspension parts, etc. The commercial chassis cars usually weigh almost double as a standard production car, and Cadillac used much beefier parts where needed. This is where the Cadillac parts book comes in handy. It will tell you which parts are the same, and which are different. If you purchase a parts book, make sure that it is specific for the year of manufacture of your car. It it is a broad range parts book servicing many different years, a lot of the specific detail that you will be looking for will not be shown. I have a Cadillac parts book specifically for 1978, and one that covers from 1976 to 1979. The one year only book has more detailed information than the 1976 - 1979 book. Both are helpful for cross referencing parts from other production years.
For non moving dash parts, such as dash cover, air conditioning ducting, etc., the cross over is quite wide year wise and the same as other rear wheel drive Cadillacs. When it comes to the electrical and mechanical parts of the dash, some are the same and will interchange, and others are not. This is where you will need to be comparing parts and part numbers to know for certain. Even parts that look alike might have different construction inside, or have different electrical values that will not work in your application year. i.e. a 1972 air conditioning controller will look the same as the 1973, but electrically wired different.
As for the engine parts, there is a wide variety of differences and similarities. A standard Cadillac might have one type of emission system that is different from the commercial chassis, however, the commercial chassis emission system might be the same as the Cadillac Eldorado's. One thing to always be leery of is the auto parts books that say that the parts are all the same across the entire line of Cadillac chassis's. An example is that the auto parts stores don't differentiate between the power brake booster. The parts and service manuals show that the commercial chassis booster is different than the passenger car booster, however, the auto parts store books show them the same. The car booster will work, but it isn't correct for the commercial chassis. I don't know what the difference is, but I do know that GM wouldn't have made 2 parts if 1 would have worked satisfactorily on both regular and commercial chassis production vehicles. Whenever you buy a auto parts store replacement part, always keep the core in case the replacement part doesn't work in the long haul. If you have the old part, usually you can find someone that rebuilds them to the factory original specifications.
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