Looks to be the C30. They now lable them as C3500, one ton (US). It is a Cheyenne Camper Special. I had a C10 Super Cheyenne that I bought new in 1973. The 454 V-8 was a tremendous underachiever, and a pig, but powerful never the less.
If you compare the 350 cid horsepower to the 454 cid horsepower the bigger engine makes considerably less horsepower per cubic inch than the 350. So, I consider it an underachiever. I don't know of any that got over 10 miles per gallon (US), thus it's a pig. The 350 would do about 13-14 miles per gallon, and had only slightly less horsepower and torque overall.
Heavy duty 4-speeds were the norm in 3/4 ton and 1 ton truck manual transmissions back in the day. 5-speeds didn't happen for another ten years on trucks.