It should have a 366 big block, that's what replaced the 348 in 66. If it was a twin screw it could have a 427 in 69. Either way congratulations on scoring it. I have a 72 C-60 with the 366 and working 4th 5 speed. I also have a 65 C-80 that I restored for my dad a few years before he passed away. Clean it up good and make him glad he gave it to you, you'll be glad you did.

'65 C-10, 65 L-60, 65 C-80, 72 C-60 & and a bunch of old Chevy cars.
Never time to do it right, always time to do it over.