I wasn't really sure how you were talking about combining them. Mixing chemistries in SERIES is OK if they have similar AH and peak voltage, but PARALLEL can be dangerous, as they will have different discharge curves which can lead to massive current flowing from the chemistry at higher resting voltage to the other one.
You have all that great space in the truck bed... You probably are sold on the underneath mounting for aesthetics, but it can be a pain to have a difficult to access battery box. One of my fears for the towd is a battery fire, which I would have to remove the body to get at, so if I were to do it over I'd cut the trunk out

and mount them accessibly. It only takes about an hour to lift the body to get access to the 28s module for maintenance, but getting to the 7s modules in the "legs" of the battery box would be a pain. I wouldn't have built this way if it wasn't a safety chemistry like a123, and I took great pains to make the power wiring path as durable as possIble, but...
-JD