The L20 and L20B are completely different motors.
The L20 is an inline 6, and was produced (not sure on exact years) around the mid to late 60's up to '71 in I believe, the Laurel, Bluebird, C230, and some other various cars.
Then you have the L20ET variant (turbo) that was sold in the early 80's as a straight 6 fuel injected and most commonly seen in the Skyline.
There is an L20P propane, but was produced in limited numbers.
None of these are what you want.
The L20B was produced in 1975 to 1980, and is an easy fit for your 620. It came stock in 620's from 75-79 and was in the '80 720. It's easiest to find in a 620, but did come in some late 510s, 200SX, 710, and 610. Not hard to find at all.