According to the Chev casting "bible" the three versions of the 400 used
from 1970 thru 1980 were casting 3951511, '70-'73 4 bolt mains/HD truck and
passenger;330817, '70-'80, 2 bolt mains, passenger; 3951509 and 3030817++,
'74-'76 two bolt main with High Nickel Block. If you can cross reference
your block casting number to one of these you can determine what it is.