Originally Posted by
Steve_sr
......He said that the bore may have opened up when the slot was cut......
Pure BS. After over a century, dimensional changes when slots are cut would be a well documented phenomenon. In any case the slot itself tells the story. If the slot is the same width bottom to top, then the tube dia. is unchanged. Keep in mind that the slot width changes 3x any change in diameter, so any dia. change would be obvious.
Ream out the old, fit the new shim, bonding with the adhesive of your choice, then cut the slot with a 32tpi hacksaw using 2 blades together with the frame as a guide. Finish with a flat file to make it all look pretty.
Note that you don't need a great bond for the shim since clamping the post keeps it home. The bond is purely for convenience to prevent movement as you fit the post.
BTW - it's now mid May. Set yourself a firm deadline to be riding this by Memorial Day weekend.