I like the Hellroaring solid state isolators.
http://hellroaring.com/ Wired correctly, they can transfer 300A.
Make sure to use the same type of battery in your deep cycle as you are using in your starting system. You want to minimize voltage differences. For example, if you switch to optimas up front for the starting batteries, make sure you use absorbed gas mat batteries when you buy your deep cylcle batteries. I don't know if the GM ones are flooded or sealed, but whatever they are, make sure you use the same type when you buy the deep cycles.
For wire size and try
http://www.wranglernw.com/ they have a wire size calculator that comes in handy here:
http://www.wranglernw.com/wire/ Their products are slightly pricey, but they've got a great selection of electrical stuff if you get their catalog.
You should be able to charge your batteries VERY fast once the isolator is open. I think you will have so much amperage that the limiting factor will be the batterys' ability to accept the charge. You may want to tap into the stock system where you hook the jumper cables up to (towards the outside of the driver's side alternator).
Here's a pic of the setup I did in my RV w/ 4 6v golf cart batteries wired series/parallel to produce 450Ah of 12v.