it should be fine if the power supply can actually output the rated amount,and if it can remain stable while outputting that wattage.
I would recommend making sure the 12v rail has at the very least 29 amps on it.more amps is better.
also make sure it has a 6 pin auxiliary cable,the 7850 needs one to run,otherwise you'll need to use a molex to pcie 6 pin connector.