I am a buyer not a buyer and then seller. I have traded off 3 guns in my life and regretted every single one. A single shot Stevens 20 gauge, a single shot 22 Mossberg and a Winchester 16 gauge double. The last one was 35 years ago and I will not do it again. If it was me, I would keep the 30-30 barrel and start saving until I could buy the 357 barrel too, or better, the complete rifle. If it takes 5 or 6 months or even a year to save up the money, then so be it. I have both, the 357 is reamed to Max and is the one I use the most, but my 30-30 barrel is not going anywhere.
Good Shooting and Good Luck