Best pic I could find of both my batts. Years ago I found a driver's side batt box from a guy online. I forget what he told me he got it out of but GM did make them for 67-72 pickups so it bolted right up to my K5. One hole needed ot be drilled if I remember correctly.
I run Red Top Optimas, on my second set now, first set lasted about 6yrs in my current setup.
I use
http://www.hellroaring.com/4wheel.php battery isol/combiner.
Check out that site, lots of great info and scematics.
There is cheeper was to do it but I love the fact I can control my batts from the cab of my rig. If I need dual batt power I simply flip a switch. If my primary batt goes dead i simply flip a switch. If my Alternator dies i know about it before my engine drains the batts.
The cheaper relay or silynoid way works too, just not as "smart" of a system as the Hellroaring setup, plus Hellroaring is 100% electronic, no moving parts (silynoid/relay).
I used 2ga batt cables and 8ga single wire 140amp altinator.
Hope this helps.
Burt