puzzled over casting numbers

This seems to be a pretty common question but Im afraid I have to ask
it again. I have found a few numbers on my block and other places that
I cant find anywhere online or in books if anybody could tell me what
Im looking at Ill be forever grateful.

Best I can tell (using a mirror and flashlight) the number on the
block behind the drivers side head is
3310010

I have several numbers in the intake but the one that looks the most
like others Ive saw is 3818803, also on the intake is "H 21 7" and GMI
with a capitol "T" under it.

Stamped front of the passenger head is 1020CUB

Any insight would be killer! thanks in advance.

Ok I have been able find that casting numbers ending in 010 is for a
350 also the CUB on the suffix points to 350. If the intake is the
original then the date code of august 21 7th year means it was made in
1977 since I found that CUB was used only three years and 77 was one
of those years. Will I be able to verify 2 or 4 bolt through these
numbers or will I have to drop the pan? Thanks guys your patience and
help is greatly appreciated.