Mike's answer applies to a '30-31 Model A because there is a large hole already drilled in the brace for the cowl light hollow bolts. But if the OP is talking about installing cowl lights in his '28-29 open cars (per his public profile), this is a different matter. Only the wooden-bodied 1929 Sedans of all flavors and the Cabriolet (plus the ultra-rare Town Car!) were set up for cowl lights. If the OP starts drilling holes in the lower cowl for cowl light arms, he may hit the gas tank.
I think he should get measurements from someone here who has retrofit cowl lights to an open body 1928-29 Model A.
Marshall