I have a 1977 chevy 4x4 vin number CKR147112...ETC.. It has been in my family since it was new so I know that the only thing ever done to it was the motor was bored and the heads shaved. It has a small block 400. Now, every site that i have been to says that the 400 was not available to the half ton truck! It also has a 12 bolt rear with 3.07 gears. I also checked the RPO decal and it verifies a 400cid engine. Any thoughts?