If you use a ball be sure that's it's a good press fit...you wouldn't want the ball to fall through into the combustion chamber#@&^%*#@%&^%$&^%(^%.
Those smog holes are cast into the head, holes (especially long holes) that are in cast iron are sometimes tapered (not straight) so that something will fit tight in one section of the hole and loose in another????
If you use a tap get a 1/4 X 20 thds and set screws of the correct length.