small block identification

Hi , I'm trying to identify my 318 small block. This motor was already installed in my 66 Plymouth Valiant when I bought it. These are some numbers that I recently found very crudely written in some notes that I had taken down at different times.
Front passenger side of block---4M3181B21381
Bottom of block passenger side---5300671431826
Also bottom of block---DN 11 02 96
To the best of my knowledge these are the correct numbers. But some things with the engine suggest the motor is from the 70's such as mechanical fuel pump. Only have one V-belt running everything. But it has Magnum valve covers that suggest that it is 1996 which is what I think one of the numbers is. When I have bought parts sometimes I look up 70's engines and other times 90's engines. Have had the car 4 years and finally getting tired of guessing. Thank you in advance.