i actually did some measureing with some mineral spirits and the 20R has about a 10 cc smaller combustion chamber,

back me up Nate  i think a stock 22 has like 9.2-1 compression ratio,  the 20/22 has 9.7-9.8 to 1 and better flow,  the ideal thing is to put 22R valves in the 20 head

with oversize 22 valves in a 20 head, maybe shaved a little with arp bolts 38 weber or the 390 holly  1 5/8 headers with 2 1/2 pipe    the cops be lookin for your