2 up seat suspension

Welding up a nice lightweight 2 up seat for the back of my sled so my girl can come along. i have a '02 polaris 550 classic and i was wondering if i should be worried about bottoming out because of the extra weight. i'm thinking about going to .405 x 77 springs from the original .375 x 77. will that be enough to take care of any problems? or do i need to look into something stiffer? if they if make something stiffer. i'm about 6'4" 250 and she is 5'8" and somewhere between 120 lbs and 150 lbs. i'd give you a closer approximation but i'm not asking her hah. together with stuff probably looking at almost 400 lbs. im not looking for race quality suspension i just don't want it on the ground 24/7.
thanks
-Ryan

ps ill have to post some picks of the 2 up once its done.

2 up seat suspension