The question I have for you is: Are you OK with the "look" of the roll marks after a cut, or do you intend to simply shave them off, as shown with the slab-sided examples shown above? If your plan is the latter, I urge you to look for a 3" barrel and compare the cost to that of cutting and re-configuring the barrel sides. You may find that it doesn't cost much more (and may even be less if there isn't much fitting necessary) to replace instead of cut, all depending on your skills to do it yourself or what your gunsmith will charge.
For example, I replaced a 4" with a 3" on a Model 64 back in 2012 for $220, $165 of that going to the barrel. In the last year, I replaced a 4" with a 2" on a Model 34 for around $50 for the barrel and a 4" with a 3" on a Model 10 for $55 (barrel) plus $150 (work plus a new cylinder.) I also have 2 of the $149 J&G barrel-less frames waiting for 2" ($95) and 5" ($115) barrels I purchased online to be added. I don't anticipate either to be outrageously expensive for the 'smithing.
Anyway, just my experiences with barrel-related modification. Good luck, looking forward to seeing how it turns out!