hot rod john, i used to have a 66 c20 with a low hump so not all of them had the high hump. the c20 usually had the 4 speeds though so that's prob why most had them. in fact some of the bigger trucks have neither, they have "no hump" like the 62 and 65 c60's i used to have. no need for measurements, the low hump are round high are more square. trevota, sounds like you are right on since the manifold situation. the motor perches would not have been moved from the v8 to the 250 and the ones i have seen are bolted in not riveted. only the bracket that bolts to block is different. sorry i can't answer what engine was orig in there, i don't know much about the #'s. maybe someone will be able to.