318 OPEN CHAMBER heads are a HUGE power loss compared to 273 CLOSED CHAMBER heads, both in compression and air flow. I've run 1.88 intake J heads on a 273 with good result.
We put some 1.88's and 1.60" in a set of closed chamber 273 heads and did just a little unshrouding. Using a SP2P 318 intake and a 500 AVS made a mild RV cam'ed 318 real responsive down low for a small Airstream's Fury wagon tow rig.
Later 273 open chambers were not any better than 318 open that we could see. The 1.78" and 1.50" valves were not up to power.