Thanks for all the replies.
I got sidetracked on an ignition system experiment but am going to think about this block repair some more while I do other stuff.
be aware that when using a reamer in any drilled hole, the reamer , no matter how it's modified, will follow the hole as drilled. if the drill wanders, so will the reamer. an end mill or boring the hole is the only way i know of to straighten up a mis-drilled hole. and being a long, deep passage, a tube repair is about the only thing you can do to fix the block in question unless it's possible to weld an iron patch of sorts over the pinhole. and that, i believe, would almost be impossible to pull off. just my opinion.
This is how I saw it as well, a tube makes the most sense.
Researching exactly what kind of tube, now.
It sounds funny, but for a time it appeared a (cast iron) valve guide might work. But then I realized I'd have to ream the hole out pretty monsterously huge, like 1/2" for that to work. So for now it seems either 3/8" or (specially ordered) 9mm stainless steel tubing will probably get the nod, and I'll have to get an assortment of reamers to sneak up on it. For now, I'm assuming a .001 to .0015 press fit should be adequate/about the same as a cast iron valve guide(?).
