Actually, the 3904351 is the big block casting number for all horsepower (390, 400, 435, and even an L88) engines in 67. The 425 hp blocks were only produced in 1966. What gets a little tricky is the stamp pad number that you posted. You posted t04i4ih. The first digit "T" stands for engine factory in Tonawanda, NY. The next four digits I will guess is probably 0414, which would be April 14, 1967, which would be the assembly date for the engine. There should be a casting date code back where you found the engine block casting number and that casting date would have to be prior to the April 14th assembly date. The problem I am having is with the last two digits, which you posted "ih". This suffix code will tell you engine HP and trans that this engine was original built for. I cannot find an IH code for a 67 Corvette. These are the ones that I can locate that might look like an "ih" code in your stamping, IT (435 hp, manual trans), IU (435 hp, manual trans), JH (435 hp, manual trans), IL (390 hp, manual trans), and IR (390 hp, powerglide trans). There are a few other suffix codes but they probably would be more difficult to confuse with "ih".
I only looked at Corvette suffix codes and if the "ih" is accurate. I don't know whether the 351 block was used as a casting for a 427 block that was built for a commercial truck. If it was then the IH could be for a truck, but that is just a guess on my part. Someone else might have better data than I on the 351 block suffix codes.
I hope this helps!
Rich