Yes, you're on the right track and you're right about the base of the ESB and ROTJ gaunts being the same. The differences are in the flamethrower, whipcord housing (if I'm remembering right), and the number and placement of hoses.
Good job on the flamethrower so far. To be honest, no one is really sure what it looks like or even what colors it is exactly. We've just got the screen images to go on now and we can barely see the flamethrower in the best screengrabs we have. There's some good discussion on it over on TDH. I'm aware of 3 different makers of the ESB flamethrower assembly (only 1 sells it separately) and all 3 look very different.