Rear Gears?

[junk4cash]

Been working on my 66 c10 and noticed the rear end is out of an earlier truck. Replaced my clapped out 230 with a fresh 292. Still have the stock 3 on the tree and noticed that on the hwy in 3rd gear it really sounds and feels like its wound up at only 60 mph. I can't imagine pushing it to 70 or 75mph. Just curious if this is normal with these older trucks or should I check the rear end gears and see what I have? I am going to put a tach in this weekend and see where I'm at. Family friend has a muncie 3 speed with overdrive that is recently rebuilt and he is looking to unload it.