Modern engines have power and torque but more expensive parts and tools needed to repair.
A 350 Chevy is about the cheapest motor possible to buy and maintain. If that is what it came with I'd replace it with another one. If the existing one can't be fixed cores are usually cheap.
You can rebuild a 350 Chevy for less than the cost of misc performing an engine swap.
Now if I were thinking about replacing the motor in a C3500 and wanted something big and cheap, I'd look for a Cadillac 500. No heavier than a big block, but more torque and often available nearly free.
