I think your cutaway chassis model might be MB300 or something like that, but yes, a B300 (which is what we had) will cover most of it, at least till you get to the rear brakes.
You want to change the Engine Oil Pan Gasket? I no longer have a manual, but I think you'll have to at least lift the engine mounts up off the chassis to get it off. Speaking of mounts, ours simply ate rear transmission mounts. But it was a 1971, so maybe they issued better ones by 1979.
It'd be a good idea to change transmission fluid/filter. I don't know if a 1979 has drain plugs in the Torque Converter. If it doesn't your pan removal will change about half the fluid. See if you can get a drain plug kit for the pan, or a pan with a drain. Then you can change it once with a new filter. If it isn't too crudded up, then you can just drain/refill with the plug and not go through the pan dropping.