akrvbob, Been a long time since I played with GM side terminals. were you able to you just reuse the same cable ends on the threaded posts on the Marine battery?
An AGm battery can be put on its side, and if the GM side cables were not long enough to stretch to the top of the battery, perhaps an AGM on its side will allow connection without modification to the battery cables.
AGM batteries are pretty much dual purpose batteries, probably as good or better than any flooded marine battery, but they are rated at a bit less capacity at the same size. My AGM is smaller than my flooded battery and is rated for about 40% less capacity, but under discharge the AGM battery holds significantly higher voltages. So while it is rated at less capacity, the AGM in my view is underrated and easily outperforms the larger flooded battery. AGMs can also charge, in the bulk stage, faster if the charging source is capable of feeding it more.
An AGM battery is still damaged by chronic undercharging, so if it fails and is not under warranty, it will hurt more, money wise.
X2 power is a Rebadged Northstar AGM battery, sold by Batteries Plus, and has a 4 year warranty and is a nationwide chain for if warranty service is required. Not cheap, but I am very impressed by my NorthStar. Holds higher voltages under load, and cranks the engine significantly faster. But costs 3x as much.
I've shoehorned a group 31 into my original battery location under the hood. It is significantly bigger than the 'recommended' size battery. I'd like to lower the battery tray an inch or two for a bit more clearance.
If the GM battery cable ends do not fit on the marine battery threaded Studs, but the cables are long enough, one can cut the ends off and use one of the various other battery terminals available. Any new shiny exposed copper should be protected from the elements with some grease, vaseline or a large gob of Liquid electrical tape to seal it off from the atmosphere.
http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&field-keywords=battery+terminal
Some AGM batteries, like the sears Diehard platinum/Odyssey and NorthStar, probably others as well, will sell a marine version and a regular automotive version. There is no Internal differences in these AGM batteries, except that the marine battery has both Automotive posts as well as threaded studs. The warranty is usually less on the marine battery too. Getting the Automotive version might be cheaper and if the warranty is longer, don't fear using the AGM automotive version for deep cycle/ dual purpose/ marine duties.