Researching the engine, there are many configurations for different applications. This one appears to be about right, with the air filter at the rear of the block.
That is a 4 cylinder, the vans had a 3 cylinder and an option starting in 1967, so it is a transplant. The alternator should have been a clue for me, as '58 was a generator... The
two-stroke, 159-cubic-inch (2.6 L)
Detroit Diesel 3-53N three-cylinder engine was available in 1967; it produced peak power of 82 hp (61 kW) at 2,500 rpm and 193 lb⋅ft (262 N⋅m) of torque at 1,500 rpm. wikipedia
