Are you sure its a rod?
When I was 16 i got a 400 small block inmy 66 chevy truck with my dads help (ford guy) (never worked on a chevy) One night I put it to the floor for about 4 miles. Man I was moving, when i clutched it it died DIED. finally recrunck and hada rod knocking. Drove straight to the shop Pulled pan and it took about 1/4" off two of the rods. Put crank kit and rebroke the motor in. 50 miles later I ran it real hard again and knock, knock, knock. Sounded like a rod again. Stoped and bought some rod bearings, went to shop replaced all (tight???) bearings. Crunk up and still knocked. Dam cut the belt and it turned out to be the water pump.
Later I found out the six cylinder flywheel does not work well with the 400. very unbalanced, I always thought the vibration was from my homeade drivesaft.