Over actual poundage, bow design is the big factor.
Im presuming you'l be wanting to use a Widow
Ill see if i have some pics of Elephant ribs, i remember having a really good look at a full skeleton they had on display at the Elephant display in Kruger.
Honestly, if you center a Rib, neither a 83lb or a 100lb trad bow is going to work. Its a bit of a lucky packet, you go through the Ribs and your in with a good chance.
but even with a great shot, the Bulls can go a long way, their heart is huge but your broadhead is tiny.
As Rob said, pic your shots well with a 50lb set-ups
We did some interesting testing this year during Buffalo Season, with interesting results on penetration, broadhead integrity, and interesting to see which broadheads really outperformed some of the big name heads. Man did we bend and break so heads