Here's a link that may help you. Look at the last picture in Post #1, download it to your hard drive and open it in a picture viewer that you can Zoom in on the info for decoding.
Your first two numbers are mixed up it should be H, M not M, H.
This stands for 72 model year and Mound Road engine plant built the engine.
340
Regular fuel.
and you're missing 3 numbers for the engine build date and sequence #. Try looking closer at the block under the left front head. Sometimes the numbers are hard to see.