I know a little something about that. There are two options I have been involved with if you have an interest let me know. One I did myself and the other was done by my Dad, much simpler and less work (Dad is very smart
). Mine was documented to some degree here...
My experience converting a cart from 6-8 to 8-6 volt batteries
That said there is a new Trojan battery called the Ranger (I think) that has about 90% of the capacity of 8 T105s.
This may be a better option if you do not wish to take on the project to make room....
... or give
jimbo2012 a ring... solar + rangers... you could drive for miles and miles and miles...