[soulman67]

You need both batteries, because that 6.2 has 21:1 compression ratio and they don't make gear reduction starters for that high of compression. If you run just a single big battery you will go through starters fast. Plus with two batteries you have more starting power which is needed to start 6.2s and 6.5s the starter needs the spin fast.
You can put a turbo set up off a 6.5 and gain power and fuel economy. There is a lot of good info on thedieselpage.com I am member of that site and that is where I got most of the mod info on my 96 6.5td and it would out run and pull my friends 02 powerstroke with a 80hp chip. and I still got 21.3mpg runing 65 on average empty loaded I get 16 at 70 that with my 88 grand wagoneer on my 24ft gooseneck. I can run with a stock 300hp d-max but that 360hp d-max will leave me in the dust.