After running it approximately 30+ miles I can smell a sulfur smell?

After running it for a bit approximately 30+ miles I can smell a sulfur smell. I have had the cats checked and was told the front one was clogged back. One was good so I changed the front one. About 3 weeks ago but before changing the cat I ran a couple cat cleaners as it directed but still no help with cleaners. So, changed it and can now smell sulfur but couldn't before. I did have to cut exhaust pipe when changing cat cause the bottom connection from cat to exhaust pipe the nuts was welded to the plate. So, when I got it out I grinded the studs off to reuse the plate welded back together and used high temperature quick steel over weld for extra sealing but like I said still smelling it just curious if it's just buildup or what won't be able to put money into it for a while financially not able to ATM thanks in advance.