draw weight and arrow weight

[OS]

It looks like I'm going to get the chance to hunt water buffelo    
my concern is since my shoulder problem I've only been shooting 55 - 57lbs.  I am afraid even with a heavy arrow this wont be enough bow!!
I can draw my 63# longbow but have a hard time with form.(and shoulder hates me after the 4 shot).  I have alway shot heavy bows 70 - 80# range but after tearing my shoulder I can't do that and still aprihensive of the lighter poundage any thoughts from anyone with some experiance with buff or other realy big game?  I'm turning my Snuffers in for heavy 2 blade also. Any help or thoughts would be great