This may not be what you want to hear or a direct answer to your question, but I'll just say that if you're serious about getting into MMA, forget about the bodybuilding. It's next to useless in this sport.
Anyway, I'd say 6'1" isn't too short to be a heavyweight. But what's also important is reach. If you're not working with at least 76", you're definitely at a disadvantage.
HW: Emelianenko, JDS, Filipović
LHW: Rua, Jackson
WW: GSP, Penn, Alves