You won't have to fatten Bruce up for UKC or Intl' shows. Some of the dogs that show and do well or win best of breed in these venues don't even look like Labs. I would guess that you could take Bruce to a show right this second without so much as brushing him and he would do very well. It is a whole different world as compared to AKC. It's like local beauty pageants at Holiday Inn vs. the one on TV. I wouldn't worry about it at all.
It's amazing to me how much weight a conformation Lab can put on before he is actually really fat. Sam has gotten up to 110 (he is 80 now) and while he was chunky, he was not even close to being a walking coffee table and he was still very active. Linus got up to 96 at a year old and he, too, is now 80. He was never what I'd call fat and he was doing field work at that weight.