Yes, you're nuts. :p I don't the the fin issue will cause any problems. Brake sizes varied some depending on the year and the model, usually the difference was with shoe width. The distance between drum faces is usually 3.5" more on the later rear ends, part of that is the housing and part is the drum. You can slip the newer differential under your truck using newer wheels and narrow tires, but there isn't much clearance between the tire and the fender.

All I can say is get out the measuring stick.

Fred
52 3600
69 C-10