06 SB
WKR
I have used Slick Trick Magnum on elk and many other heads on deer.  It is important to have a very sharp broadhead and hit where you aim.  I personally would use either a Slick Trick Mag or a Micro Hades.  Practice at 80+ yards if you can to make a 40 yard shot seem easy.  50# will work just fine but you will need to get good estimating range in case you do not have time to use a range finder.  If you do not have one of those, buy one.