I have a 742 83/4 rearend with spool and moser axles. in a 1968 dart. supension cal trac bars and mono springs. will this rearend hold up to 700 horse from a 500 inch big block leaving on a trans brake at 4000 rpm. really need some advise thanks  

For the rearend to survive you will have to brase the housing across the back. The axles and spool will have to be at least 35 splines.
A large factor will also be how much torque the 500 inch motor has at the 4000 rpm range, and the size of tire you use. If this engine is making anything close to 700 ft. lbs of torque in a heavy car you will want to upgrade to a professionally built dana or a  9 " ford rear end.  

Thank you I do have 35 spline axles and the rear is braced. I am running 28 inch tall 10 inch wide tires. The car weights 3200 with me in it.  

Flip a coin, but I can hear the time bomb ticking from here. I'd go with a dana 60, it's cheaper in the long run. You lose the rear and many times there is a chain reaction. Driveshaft *** transmission *** putting it into the wall $$$$$$$$$$$$$$$. Dave