It could very well be a C40. GM did use the same sheetmetal from 10-40 from at least '60-'72. One way to tell for sure is to count the number of luggs on the wheels. A C30 will be 8 lug front and rear, the C40 will be 5 lug in front and 10 lug in the rear.
The 351 engine is a possible givaway, too. The 10-30 trucks only had the 305 as the sole engine, the 351 was only used in 1 1/2 ton and larger.
Can you post a picture? That'll really help with the ID.
