Electrician question.

I was installing a 14-50R outlet to replace a 6-50R outlet. I also rewired it (to 6-3 tray cable) and the 14-50R outlet appears to be too big for the box. Is a different box required or am I just not trying hard enough?

As a side note, I had the outlet wired with 8-3 Romex on a 30 amp breaker and when I removed the Romex there were what appeared to be burn marks on the paper side of the insulation, but no evidence of damage to the Romex sheath or wires. I counted myself lucky and moved on as I was installing larger wire for the same load, but it also made me wonder if there was some reaction between the romex sheath and the insulation paper because there was no evidence of overheating anywhere else. Any ideas?