400 big block

I looked up the 251s; they are for a 3.75" stroke with B rod length (6.358") in a 400 block + 060 overbore. So, they will work with stock rods, but not stock crank unless you're turning a 440 crank down to 400 mains.

They will provide the "squish" you're talking about, but only with the right head gasket and proper deck clearance. Summit's description says there's a .260" quench dome...that's a helluva dome for that relief in the heads. So, make sure your pistons are set properly in the bore with the correct thickness head gaskets, otherwise you might not have enough clearance to run them.

452s do indeed have the same chamber as 906/346 heads (manufacturing variances aside)...open with a quench relief. There's someone on here with a pair for sale if you need a visual...or google it...or eBay search...any number of ways to verify this.