Howdy and welcome.
You can look up the casting numbers on the web. That casting head number you gave is for a 69-70 Big Block, 396, 402, 427 and 454. Rectangular closed port with 109 CC chambers. Another sight has them as 69-72 and for crate motors from 70-72.
Motor was built in Tonawanda, 03=March, 20=20, LD is 1969 Chevelle 427, 425 HP manual trans, special high performance.
http://www.nastyz28.com/chevy-engine-code-stampings.php