[MENTION=170517]rccardr[/MENTION] - I think it was your thread that got me thinking I could do this!
Modification: Old style Shimano downtube shifter mounts to later style

I was going to drill through into the down tube first with a very fine bit to prevent having the tap bottoming and breaking. I get that I have to stop after every 1/4 turn, reverse and clean the threads and that I must oil the tap.

Do I not still have to very carefully file tabs on the bosses to allow the replacement bases to fit?? Or, are you saying that just re-tapping the threads will be all I need?

I saw the mention of using the .5 inch hole saw and thought if I did this after drilling it my be neater and have a more flush fit to the frame
https://www.homedepot.com/p/Milwauke...0511/204994403

The Ross currently has the Shimano Z401 shifters and I wanted to switch to Exage Sport Shifters, if only because I have the same model levers, FD and RD on the bike already.