Putting 340 intake on 273 heads??

What pistons are in the 340?

The earlier 340's have pistons that are proud of the deck. The '68 I built with KB 243's has the piston's .018" over the deck, no need for closed chambers to get compression. In fact, it's not hard to run into piston/valve clearance issues.

Even on the later 340's, if the pistons aren't buried in the block you shouldn't have a problem making compression with 340/360 open chambers, should only need a small cut on the heads. Otherwise you're going to have a big mismatch on the ports.