I am in Alberta, so all I can offer you is information;
(also, I am typing this from memory, so it's possible I might get something wrong)

1) the 455 never went through lightening like the 400 - a 1970 block, is no better than a 1976 block from a performance/building standpoint - the only difference is that the earleir blocks (up to 1973) were often drilled and tapped for four bolt mains - even though they were finished with two bolt caps. The only exception to any of this is the SD455 block - that one was different.

2) as far as 400's are concerned, the 1971-1974 988 block is generally regarded as the one to look for, as they are the most plentiful and were not subject to lightening.
The 557 block is the lightened one, but sources which have more first hand knowledge than me say that it can still easily handle 400hp.
the 988 block was recast after problems with the 557 block showed up, and this was the casting used for 1978/1979 W72 blocks.
The 557 and late 988 blocks may be missing the early or full sized car motor mount bosses.

I hope this helps!!

https://www.kijiji.ca/v-engines-and-engine-parts/kitchener-waterloo/400-pontiac/1478574336

Here's a good deal in Kijiji , if it's an early 1975 988 casting it will work for you ...