GAA as an engine serial number prefix IDs the block as:
1949 passenger car, regular engine (216?), Flint, MI
http://chevy.oldcarmanualproject.com/chevyresto/4908.htm
http://chevy.oldcarmanualproject.com/models/engine.htm#49eng

This is not an expert answer - just a wild guess: it could be a low-pressure 235 head on a 216 block with holes added for attaching the short side cover?

What is the head casting code and head date code? This info might clear things up and/or make the guessing more interesting.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.