"The fire is back in the belly (to box) and obviously I want to have a pretty good year this year in regards to rugby," Williams told AAP. "I know this will prepare me not just physically, but mentally as well, for a massive season."
Williams, who has played 23 Tests for the All Blacks including the 2011 World Cup final win over France, switched back to union late last year, appearing in New Zealand's last four internationals of 2014.
He wilted in the final rounds against Botha, but is confident he will have enough in the tank, if the bout with Welliver goes the eight round distance.
"There's been a lot of work on defence, just getting a good seven, eight week preparation under my belt, with a lot of sparring and training," Williams said. "That's definitely put me in good stead, fitness-wise. I've got no concerns about being able to go into the late rounds."
He said a fight with Gallen (2-0) was realistic if both won on Saturday.
Gallen fights mixed martial arts exponent Randall Rayment (1-0). He won two brief and wildly entertaining fights in which he threw caution to the wind, after getting tagged early in both bouts.
"It was sort of like the red mist fogged over and he went for it," Gallen's trainer Graham Shaw told AAP. "I've got to try and cut that back and he's just got to be relaxed and box.
"We want to walk away from this fight happy with ourselves and other people saying 'geez, Paul Gallen looked like a boxer.'
"I'm hoping he's just going to be a little bit smarter."
Shaw said Gallen had sparred experienced heavyweights Bowie Tupou and Solomon Haumono and was much better prepared than for his last fight, when he spent time in hospital in the week leading up to the bout.
-AAP
