Personally, I wouldn't worry too much, and just enjoy what you have.
However, if it really bothers you, then things to look for:
1) they (meaning most railroads) started removing high ladders and running boards (roofwalks) on boxcars. All of them weren't gone yet, but they were less common. If you remove about half of them, you're doing pretty good. Pull them off, apply some green Squadron putty, and sand and file smooth. Weather the roof, and there you go.
2) 40' boxcars were starting to be phased out, because the gross rail weight limit was raised a few years before, so the older, shorter cars were beginning to be phased out and scrapped.
3) I think live stock cars were not used very much after the late 1960s, but several roads kept them for special loads, so you'll have to do some looking around.
4) The 1970s saw an introduction in exterior post boxcars that replaced the smooth side boxcars of previous years. Railbox was the leasing company which helped usher in these new designs.
5) Unibody tank cars (modern tank cars without a platform around the perimeter of the car) were becoming more common, and the older design less common.
6) Roller bearing trucks were replacing and being replaced on older cars, since friction journal trucks were being regulated out of railroad liveries. That NYC gondola you have in the last picture has friction journal trucks. Now, you can still keep them, but just something to be aware of.
The big problem I see with most rolling stock now is that often incorrect paint schemes are applied to a model. For example, the error could be in the road number, or it could be that the herald design (logo) was not done on a car of that type, or that railroad didn't have any of that type of car at all. I have two Athearn 50' plug door boxcars, based on a Pacific Car and Foundry design, but painted for the Southern Railway. The Southern was pretty exclusive with Pullman Standard cars, especially the Bessemer plant, but I have not found any photographic evidence to show me that the Southern had the prototype in question in their roster. This error is called a foobie, and when you really look around, you'll find them everywhere, to varying degrees, and, if you are a purist, it will bother you.
We must remember at times that we are not always able to have 100% accuracy, as the model itself is a representation of a real thing. I think that instead of striving for that unobtainable, we can take an artistic license in what we make, since in the end, it is your railroad.
If you look at the stenciling on the car, you can tell the era rather quickly.
If you face a freight car on one side, look at the lower left corner. There are three lines (CAPY, LD LMT, LT WT) with weights (in pounds) next to them. After the early 1980s, the railroads stopped applying CAPY and its weight next to it. As cars were reweighed and shopped, the CAPY information was painted over. CAPY refers to capacity weight, LD LMT refers to the total weight of the car fully loaded, and the LT WT is the weight of an empty car.
Next to this data is the AAR designation, which is usually a two or three letter designation. XM, XL, XML, XP, etc. were for most boxcars, while covered hoppers were LO, gons were G, GB, GBS, etc., flats could be F, FBS, FB, etc. Open hoppers could be HT, HTS, etc. And so on. Below is the date either "NEW 5-71" or "MC 6-73". New in May of 1971 is when the car was built. MC is the shop code for McComb Shops (Illinois Central Gulf Railroad) in June 1973. The car, when shopped, could have been just painted, or completely rebuilt, or upgraded with new brakes or trucks, or just reweighed. Obviously, if the date code is NEW 9-89, then the car is too new for 1978.
Somewhere on the side of the car may be a ACI plate, which is a rectangular colored bar code which allowed for optical readers to scan the car, and enter it into the system. The technology was first fielded in 1967, so anything built or painted between 1967 and mid-late 1978 would have these plates, including locomotives and cabooses. I will note, though, I can't find consistent evidence that the GM&O put ACI plates on their cabooses, but they did on their locomotives, on the stancions along the long hood. The ACI plate is left off a lot, and I'm not sure if it is because it requires the model railroad manufacturer to use more colors in their paint, or if they are working from some picture that didn't have an ACI plate, or if they just omitted it so that the car could be used earlier or later than that 11 year window.
On the lower right side, you'll find or should find dimensional data (the size of the car, inside and out) and there may be a black square with white trim. This black square is called a COTS/Lube plate, and it contains information about when the car was lubed, as well as other maintenance performed. When the COTS plates were first put on cars, they were either one large square or a small square. In the early and mid-1970s they needed more space, so the COTS plates had two sections, or divided into two plates if on a car with exterior posts. In the early 1980s, COTS plates became smaller, and had 3 sections on them, and often included the build date of the car. If you don't see a "NEW" date on the left next to the CAPY line or only see LD LMT and LT WT on the left and a 3 panel COTS plate on the right, then the car was painted after the early 1980s, and is too new for your era.
Photographs and information from the historical society will help you in your quest of not only picking appropriate rolling stock, but also in modeling your own rolling stock.
The pieces I have gathered so far were all (expect two pieces) sourced used on eBay for cheap, in lots, so I have a few more not shown that won't be used. All that end up not working for the trains intended will just go back on eBay as a single lot...so no worries if we rule out a whole bunch of them. 
