Its aways better to stay with the originals anyway.
You got that right!
Problem with GM trim is it is either indestructible (within limits; for say stainless steel pieces of spears and window trim), to easily repairable (with aluminum that can be hammered out and buffed out again), to pot metal parts that corrode; pitting on their return to the natural ores that the individual metals that comprised them over time. You can lead fill the pits and rechrome the parts if they are big and smooth such as a headlight bezel, or tail light housing, but you have no choice but replacement parts for small intricate scripts.
Biscayne, Bel Air, Impala, Caprice are differentiated by the trim level, includinf interior materials to the amount of things you don't see such as sound deading and insulation to keep the cabin temperate (includes under hood insulation) to amenities like a mat in the trunk with a courtesy light. There were also different roof lines and trunk lengths across these models so they can not be totally recreated (upgraded) by just buying fancy trim when it comes time to restore the vehicle to concours quality. So changing the grill and adding chrome doesn't change the model in the eyes of many who know better.
I believe in Updating a car to modern standards where possible (there are not as many choices with a full size Chevy as there are for the more popular Camaro or the Chevelle). But with a GM most parts interchange (Corporate policy was to make one part and reuse it over and over again). The Corvette was just a bob tailed full size car up until 1964, when GM introduced the Saginaw steering gear box. So on early Chevys if you can not find a part you want reproduced look on a Corvette restoration site to get that same part (at a much inflated price). Keep in mind that a Cadillac is just an over priced Chevy with many parts interchanging (but once again you are going to pay a lot more for the same part because Corvette and Cadillac owners have deep pockets and the manufactures know it).
Because the full size is nearly half again as heavy as the smaller Chevy models things like ball joints, brakes, and tie rod diameters are larger on the Full Size. But they still share common parts with the Corvette and light pick up trucks. GM took the raw casting for a brake drum and drilled five larger diameter studs (or six studs) in the wheel hub and drum on a pick up, but the Full size on older models had shared the smaller five on 4ĺ inch bolt pattern to match the wheels of other GM cars. Pick-up and Full Size differed only in machine work on many parts (so if you are willing to pay for a machine shop to work on it you can make it fit your car if it is a rare part). That is why I chose a pick-up radiator when it came to cooling my big engine out of a truck (it wasn't a bolt in I had to modify the lower mount lowering it to get the hood to close) but that is what hot rodding is all about. Just remember if you decide to use a disc brake off of a pickup or Covette to also use all of the parts that were on the donor vehicle (such as the master cylinder, proportioning valve off a truck because the 'vette doesn't have one with four wheel discs, and the residual vale). Bigger brakes such as 12 inch or bigger rotors will not fit inside a 14 inch wheel, so originality will suffer as you get a safer vehicle.