400 Block...what to look for?

Any 400 block will be ok. The overbore thing is a huge myth.

Don't listen to anyone saying "get a different" motor. 400's have serious potential. My '65 Newport has one and it suprised me-even in that boat!
It does have 452 heads and a Hughes camshaft though.

Somewhere around the mid 70's, they started the figure 8 cooling passages and I like those for the extra cooling capacity. That said, I'd build any 400 without hesitation. I have one that I am going to tinker with when the funds allow.