Easy to build up muscle under a costume... are you going to be shirtless, tank, or full costume? And don't forget, creative "make up" or body paint can give an illusion of musculature...
I think you should try to bulk up at least a little bit and I agree with pooper, you should get grow a little stubble that would really contribute to the costume!
I think your face resembles wolverine from the comics more than Jackman or maybe its just me...either way good luck man!