It appears to me that a good size is 5/16-24 threads and 1/4” length. This is the size that Marine Crankshaft installed on my 3.0L SC crank and they fit nicely. Since they’re not tapered pipe thread plugs, I would suggest thread locking compound to ensure none come loose AND it should help you resist the urge to overtighten them
And since Neil mentioned some of the things not to do, I’ll ask his thoughts on cross-drilling the crank at the number 4 main journal. Doing so intersects the oil delivery galleys inside the crank for rod bearings #2 and #5.
These two rod bearings are the last to receive oil since oiling originates from the ends of the crank via #1 and #8 main bearing locations. In order for it to be most effective, the oil holes in the #4 crank main journal, #4 bearing and #4 main bearing engine case oil galley need to be enlarged. You also need to groove the bearing (or the journal itself) to ensure oil from the case enters the crank and delivers oil to the #2 and #5 rod bearing galleys as intended
I’m not implying this is a must. Just speaking from experience on an engine run low on oil. Those middle rod throws on the crank are indeed the first bearings that get smoked when the crank is not receiving all the oil it needs. Not saying this is a solution for insufficient oil fill volume. Just an extra level of protection so to speak