This is probably the best thread on adding batteries on the E Series frame rail. LOTS of info! My post#40 has the way I did it. Ramsey (RIP) mentions Agile has a dual battery setup that can be purchased. I am not sure if John is still offering it.
If you are going to to the trouble of working with two boxes to add Ah then go to two Grp 31 AGM batteries. you can use the mounting ears from your box and have a battery tray fabbed up to securely hold the two batteries. They will fit laid on their side and will tuck up well. if you are not worried about ground clearance then they can be mounted in the up right position.
If you decide to go w two grp 65's in the stock boxes, you will likely have to reposition the current box to make room for the second box. This will require drilling some new mounting holes in the frame. Pep Boys carries a Bosch Platinum Grp 65 AGM that is around $180 when on sale. It has 65 Ah capacity. A Grp 31 has around 100 to 110 Ah.