It is an excellent GM block.
It's probably a 1972 350.
4bolt 350ci
Gm 3970010 the model?
it is a 1969-1979 motor it was used in cars, trucks, and vettes
A81 should be January 8, '71,'81,'91.
It's a 4" bore block. Has been used for 302, 327, and 350 engines.
350 and 327 [1966 - 1980]
1968------327 & 1969-79 ----350
GM engine block # 3970010 was used as early as 1968 on some 327ci engines. From 1969-1979 it was a 350ci engine. It could be a 2 or 4 bolt block used in cars, trucks and even corvettes. You can also find these blocks with a "MEXICO" stamp on them near the block ID. It has been said that these are high nickle content blocks and may be stronger than others.
It's hard to know for sure, but either the cam is pushing the valves open too far, the pistons are sticking out too far, or the cam timing is off.
those numbers are the date that it was built k=month .... 24=day ..... 5=year November 24 1965 or 1975