the number should be stamped just above the oilpan, on the passenger side. It should start with 336. There are 2 different types of 336 motors, so the number after will identify it as an early 336 (based off the pontiac 370) or a later 336 (based off the pontiac 389).
I had an early 336 in my 59 gmc 1/2 ton longbed. I changed it out for a 59 pontiac 389 that I had bored and stroked to 447 inches. The 59 389 is virtually a drop in replacement for the 58 336. I had to have my flywheel bored to match the larger crank flange.
