welcome to FTE!
well you said it was a 1975 camper special. that means it should be an F250 which would rule out 302. also it should rule out 351M/400/351C.
I know it's just me, but I always thought that the 351C was part of the 302/351W engine family......maybe I always thought wrong.
the easiest way for us to help you would be for you to post the VIN #.