The rotors should slide on easy, so it sounds like you have the wrong rotors/bearings or the wrong/incorrectly machined spindles. You might want to contact Heitds to make sure they sent you the right parts, and also to double check on the recomended brake components to use, as "78-88 Chevelle" can't be correct, Chevelles were discontinued in '77.
You mention getting the spidle nut "real tight", you shouldn't tighten them down very tight at all, as it will damamge the bearing races. Should only go to just snug, back off, the lightly tighten it to the first cotter pin slot.
