Don't know motor size?

[GMC"ER]

I just bought a 69 gmc stepside truck my first! The guy that I bought it from didn't know what engine it had in it all I know is it is a V8 with automatic trans.? I have looked online and still cannot find the size. The block number on the drivers side on the back side of the engine behind the drivers head is 3970010 I found a list and it gives a 327 and a 350 the same numbers? Also the number in front of the passenger side head is V0909CHH and 385117462. Does anybody know the numbers and what size engine I have? This is not the original motor it had a straight 6 in it originally but auto trans is original. Thanks for any help, Robert