Here is how I did it using a 275 fount and a 200A valve stem on up assembly.
One should use Safety glasses and face masks.
First find you first victim. A Coleman 275 that hasn't been used in years.
Tear her down. I even pull the check valve.
I will flush out the fount with water to get any residual fuel out and heat with a mini torch the area where the valve stem treads into the fount to break loose the sealer Coleman uses.
Than I lock the valve stem in the vice. Not too tight.
Put a rubber drawer liner on her to get a better grip and spin her off.
This is where you want to cut the valve stem off.
I use my Dremel cutting disc.
The added length gives you room to grip it with Vice Grips to re-thread it back into the 275 fount.
Stripped the paint.
Got her ready for painting.
Put a little sealer on the valve stem threads.
I made the mistake of painting before threading in the cut down 275 valve stem.
Than cut if flush with the fount with my Dremel cutting disk.
The inner opening doesn't need the hole re drilled as it is very close to the size for the tap.
The next picture come from Saltfork at the Coleman Collector Forum because I forgot to document this part.
The 200A fount is 1/2 inch taller than the 275 fount so you will need to trim the 200A FA tube.
To be continued. Need a coffee break.
Edited for accuracy.