I hope you meant 1/2'' drive instead of 1/4" drive ratchet ? A 3/8" Allen head is smaller than a 10mm Allen head ... too bad . A T55 Torx is too small to help also. Are you using a GOOD quality socket on this bolt ?
If you have a heat gun , you can apply heat to this fastener head for about 15 min. & then use a breaker bar and proper length extension to get away from the exhaust pipe. Push in against the fastener while someone else holds the bike (so it will not fall over & leave you greatly disappointed) so you will not push it over. I assume you are turning this fastener CCW ?