It's hard to tell exactly where you're locating the battery under the front gaucho. I've attached a picture of the framing from below the trailer. The part painted white here is the exposed part of the frame, as it enters the bellypan. Notice how there is a wide area that is unsupported in the center, but see how well supported it is outboard. Unfortunately, this will drive you to place the battery further to the street side so that it sits on the frame. Stated more simply: eyeball the frame from the tongue and try to locate the battery on that frame.
The other location you're considering is well supported. See post #15 of this thread: http://www.airforums.com/forums/f145...vel-45046.html
Notice the two outriggers just forward of the wheel (the tongue of the trailer is to the top of that picture). If you locate it here, you'll want to orient the battery sideways so that it spans those two outriggers.
Weight distribution is key, of course. Have you considered weighing the trailer to see what your tongue weight is now? Our local scale will allow us a second weighing for a dollar. If you weigh the entire trailer, then hook up and weigh again with just the wheels on the scale, you can calculate the % of weight on the tongue. It should be 10-15%. If you are close to 15%, you probably want to locate the battery further back.
This is a heavy battery, and either location will affect side to side balance. You can also consider 2 6-volt batteries and locate them so that they balance well side-to-side. We used the Lifeline GPL-4C's and are very happy with them.
Also, you don't have the original axle, right??