13:1 Compression Stock Stroke 400??

You could also get a piston or do a combination that puts the piston above the deck. My first race engine was a 12 to 1 compression 302 SBF that had flat top pistons in it. I had the pistons .023" above the deck to get that small cubic inch engine to make that kind of compression. While it sounds a little on the crazy side it worked without issue for years. If I had to have 13 to 1 from a 400 I myself would not be interested in using a big domed piston to get it there. Decking the block and milling the heads would without question be used as well.

Tom