with either of the two options above, you are talking about greatly increasing the load on the cart by 3 people, you mentioned 5 people on the cart. for starters you are adding a rear seat (more weight) plus the additional people.
IF you want this cart to be reliable and do its job and not leave you stranded (it will happen sooner or later) what you should do is the following:
Upgrade to either a 400 or 500 amp Alltrax controller
HD F&R switch
SD 400a Solenoid
(^^^ the 3 items above can be bought together as a kit from
cartslimited.net)
2 ga. cables (ALL OF THEM)
and make the jump to 48v pack, which you can do by replacing the 5v batteries with 8v (they fit in the same battery rack. by going to 48v will give you a 33% jump in TORQUE.