As far as physical fit the 4 pin modules were all the same and were used in all the 6 and 8 cylinder Gm's from 75 to 80. Some went slightly earlier and some a few years after. Coils were all physically all the same for the coil in cap stuff. Caps obviously you cant use a 8 or 4 cyl ones. There were also even and odd fire 6's. The odd fires were the early Buick v6's and the Chev 4.3's. The caps for the later buick's and inlines are the ones you would want. For all this stuff there is a range of quality in the parts stores. Since modules and coils are the same in all you have your choice there to buy what ever quality and brand you want but in the caps since the 6's are not all that common you may not have as many options, you may be stuck with the store brand. The big names like MSD and such all make the modules and coils and V8 caps. Never looked to see if you can get a 6 cap from them, I know I have never seen one stocked in a store. Unless you are broke down it may be better just to get it from CI since you know he is not stocking the cheap stuff.
Pick something safe in the middle like a 78 chev 1/2 ton pickup with the 250/6 in it. That will give you everything you need if you choose to go the parts store route. Trucks are safe since they did not tend to have as many emission options as the cars did by then. Even the wire set should work.