Yeah 70" is a tad long. I shoot my elk standing up with my back to a tree. If you do that you're fine.
My buddy likes to shoot from his knees, however he shoots a 58" recurve. 70" from your knees would be tough without really canting.
Regarding poundage and arrow weight... I'm in the heavy and heavier category.
I have personally been involved in several woundings (mostly my partner
) Penetration or shoulder hits were the issue every time. A mature bull IS thick skinned and heavy boned.
This has been debated repeatedly on here and most agree that bulls are easy to kill with double lung shots and very difficult to kill with anything less.
Good luck. (start training physically NOW!) You'll thank yourself in Sept.