When the motors were built at the factory back in the day, the cam bearings were what they called FIP (fit in place). As the motors run and season, or stress relieve, or relax, or whatever term you use, the cam bearings would wear-in over time with the stress relieving. ALL cast iron will shift its grain, and can twist, bend, cup, etc..
Anyway, when new cam bearings are installed, you are installing precision sized bearings, and if the block has twisted the cam tunnel, the cam will likely be tight. You have to go in manually and scrape the bearings to fit. Another thing you can do is to cut a diagonal slot in the bearing journals on an old cam and run it in the new bearings like a reamer.
I usually have no problems with Chevys, or Fords, but for some reason, Mopars are usually more contankerous with snug bearings. Just take a small pen knife, and scrape the highspots on the bearings. Another reason is the actual bore has galled or raised metal around the oil hole, and this can make for a high spot when the cam bearings are installed.
