I don't know the size exactly- but somewhere I read the pre-drill spec on a parts site..I believe that LP uses a standard size Venturi. Will try to track it down.
also, I took my bits & used the smallest size that would not fit through the existing hole...the bit is so small that I can't see any etched size... Unscientific I know,
But I got the idea from someone who did the same hack on a jen air grill. I figured I could always just replace the part if I screwed it up. Very happy so far. Easy enough to tame the bigger flame with knob.
I really like the 2stone. My wife split it with me for my b-day, so I jumped on it!
I will post some pics soon. Thanks guys for replying.