Actually, I'd use my valve seat and guide machine with the air float table to position the guide precisely under the spindle, and bore the guides out to a thin sleeve, then ream the remaining material out. I have an assortment of chucking reamers in .001" increments that can cut guides away without enlarging the bores in the head. That machine minimizes the error when installing new guides and also makes it much simpler to machine the valve seats to align with the new guides. Sometimes when the guides are installed with hand tools, it takes quite a bit of seat grinding to align the seat with the new guide. That machine and the associated tooling sold for about $15K when it was new, but I was able to purchase one used for about $2K. Ditto for my crankshaft grinder- - - -I bought two of them for $3K, a running machine and a disassembled one for parts, for a total investment of $3K. The same one in "as new" condition usually sells for $20K or more.
To answer your question about trying to make the guides move before drilling- - - -it doesn't hurt to try, but it's a crapshoot about whether or not they will move. Just don't try to force things.
Jerry
