Knuckledragger
blowing chunks and grabbing porcelain
That engine in question is a 6.9. But yes I believe all IH IDI blocks have the double hole thing in them. The front one under the fuel pump is almost always plugged with a lead ball.
The silicone thing worked to seal it up for a while but I've got one heck of a leak in that engine right now and I'm thinking its that hole again.
Sean,
Like others have said, if the hole is clean, the RTV will work. Unfortunately, it is very difficult to get it super clean, so the next thing to do is thread the hole and put a screw in plug in there. I think it is the right size for a 1/8 NPT pipe thread, so you can use interference to seal it up. If not, use any straight threads that will work and a copper washer under the head to seal.