rebuild and bullet proof question

my 833 4 speed has proven to be my weak link since I put in my 416....seems it wont take the higher RPMs going into third...she pops out every time..she did come out of a car with 43k on her and and a litlle stock 340....now the 416 is just a tad more HP and TORQUE..a few of my buddies say its time for a rebuild and bullit proof it....all you HIGHER HP guys have any suggestions for me on my trans...

thanks any and all help will be greatfull

Wayne