Found 340 block built May 5, 1973

Looks like serial # 3G212185 (on passenger side pan rail)
Looks like serial # 3M340 X*423829X* (on driver's side of block under cylinder head)

X* = I can't determine if there is a digit in those locations or not--hard to read.

To the best of my knowledge:
3 + 1973; G = St. Louis, MO final assembly plant; 212185 = last six numbers of vehicle VIN

3+ 1973; M = Mound Road engine assembly plant; 4238 = engine built 03/05/1973; 29X engine built that day

It's hard to read that number from the front of the block, can you get more angles of it so maybe the light reflecting at different angles may help us figure it out...

The only way to read it is to blow it up, try clicking on the foto and see if you can use a zoom feature.