Knurling the guides & pistons are just a cheap way to temporarily fit/fix things. Doing this to the guides helps promote lubrication of the guides & valve stems, but it does not last long.
I have not checked lately but last I did, bronze guides cost about $10 each installed? Probably more now.
If you could make it a 12 port, you are probably going to adjust your fueling tables as it will not be a Siamese port any longer. But since the gap is not too big from the factory, it might not need changing at all? Let us know your findings. Easy to do, just a heads up.
My 4.3 Chevy V-6 heads have a bad gap also. Please excuse the non inline pics.