do i need to change the rear end of my 86 4.3 to handle the hp of my 400 sb?

It depends on how powerful your 400cid is and if your tires will hook up on a drag race start. The 7.5" rear that you have now is the weakest ring and pinion that Chevy has made and was never intended to handle much more than 300lbs of torque. Put enough torque thru it and with sticky enough tires and that rear end is history; usually the spider gears break first.

Hook up a Ford 9 inch and you won't have to worry....