You would be an idiot to try to ream the headtube to try and make the headtube accept the "long time industry standard 22.2mm" stem RATHER than the "1966 & LATER SCHWINN STANDARD 21.1mm SIZE".
The whole reason that SCHWINN introduced the thicker walled, strongest headtube in the industry in 1966 is because that added strength according to Schwinn engineering made for a better handling, and safer bicycle.
Now you want to take your ROSS, which obviously saw reason to construct this model to the stronger, thicker walled headtube like Schwinn,.......YOU WANT TO REAM IT SO YOU CAN FIT A MORE ATTRACTIVE LONGER REPLACEMENT STEM............serious dumbass move!!!
Sure, there were not many manufacturers other than 1966-on Schwinn and some of the Panasonic Japanese Imports that Schwinn labeled and badged in their lineup
You'll have to search a while for a possible aluminum stem from 40 some odd years ago that meets the extended length that you need. There were probably a few quality aftermarket 21.1 aluminum quil stems in the extended length that you desire. Given that quill stems have been gone for decades except on inexpensive beach cruisers that you might find at Walmart, you're not gonna find new anything at local bike shops unless you can jump in Ziggy's time machine and take the Quantum Leap back to 1982. Keep looking, you'll may find something after a while.... Don't try to ream the headtube as that is just beyond stupid! You would be better off having a machine shop reduce a 22.2 diameter stem to 21.1mm diameter using your existing 21.1mm stem as a guide. I don't recommend that because the cost to do so might not be suitable to you, but a professional machinist could possibly get a reduction of such a part that would retain its strength and integrity. Maybe some cat you went to school with has a machine shop, or that cat's son has a machine shop, or your son-in-law that builds race cars might have a buddy at a machine shop. It is fairly easy work as long as the reduction leaves enough meat on the 22.2 stem after reducing it to 21.1mm. The machinist should be able to immediately eyeball the 22.2mm part along with your existing 21.1mm part and tell you definitively, if it can be done or not with that particular 22.2mm stem--------reducing to 21.1mm. IT MAY JUST BE TOO RISKY WITH AN ALUMINUM PART of UNKNOWN ORIGIN & INTEGRITY.........I haven't yet investigated it enough to accurately say.
You absolutely don't wanna take chances with your steering controls.
Have you looked at possibly using a stock stem from a Seventies era Schwinn Continental, as that would be aluminum and 21.1mm? The only question would be would it be useful in the length department or would it still be too short.