Hey guys, I have been working on vehicles longer than most of you have probably been alive (helped build a 23 T-bucket back in 1984). Then you start adding in the fact that I have spent my whole life around mechanical and electrical stuff (I work in a power plant), you learn things from all sorts of people. Now, mix in a healthy trait of wanting to try anything and you get a dangerous combination.
Panda, as for your setup, sure you can mount the battery in the back, but I will caution you that you should in that case use either a sealed battery or a gel/coil style battery. I say this because when you charge a battery, it can develop dangerous gases (hydrogen, sulfuric gas, etc) and if you have it in the cab of the truck, it may not end good, especially if you are deeply discharging the battery and using the truck to recharge it (high charge rates only make the gassing problem that much worse).
A common trick for our trucks is to go with say two 850 CCA batteries and mount them sideways and then you can mount both of them in the factory battery location. All you have to do is move a single vacuum canister out of the way (gotta keep it installed else the truck runs like hell, but it does fit in a nice little pocket right below where it gets unbolted from). I normally use a piece of say diamond plate and bend it at a 90 degree to be a bracket for the batteries so they don't slide into the engine and that also makes for a nice place to mount a battery isolator block.
I am not sure what type of isolator that tonydiv has, but you want one that looks like a car audio amp, not a relay. I say this because it makes recharging the batteries "idiot proof". I am not saying that you are, butt...... LMAO. The battery isolator is essentially a fancy diode complex that also controls the amount of charging going to each battery. But, when the truck is off, the isolator maintains the two batteries completely isolated from each other. So, you can't accidentally discharge both batteries by accidentally leaving the switch turned on for a relay or leaving the key in the RUN position to keep the radio going. It is also easier on the alternator as it won't be drug down by the completely drained battery, which can lead to early alternator failure (overheating of the rectifier diodes or melting of the field windings due to too high of current).
Granted, if I get my F-150, look out. It is going to make Nukie look like a play toy. He he he he. I have lots of plans for that. The trickest one will be using the factory key fob to pop the doors after removing the door handles. The doors will pop independently. Oh the joys of understanding modern day electronics and knowing how to modify them. Then add in a custom air ride suspension and now we are talking. I'm also looking at doing a retro 70's look to the truck. I will see if I can post up a pic that I played with for what I have in mind.