crosswind air gap or stock 4bbl manifold

That cam seems to me to be a pretty small cam at 204-214 @ 0.50"
Can't see it effecting DCR that much.

The air gap is definitely gonna give more top end
Maybe with a little loss at bottom end. On 340/360 they don't seem to sacrifice much bottom end but who knows on a 273.

Your build right now is at the border what I'm mean by that is everything you pretty much do will trade bottom end for top end like if you go up in cam, carb size etc..

So you got to decide what you want.