Jeff, your Triad is also known as a Univolt, aka a buzzbox, or electric boat anchor. One of our forums members (68 overlander) sells converters, you can contact him for more info.
A group 27 SHOULD fit in the spot, I've forgotten the measurements for the 27, I think it is 2" longer than the 24. If you have 2" of space between your 24 and the battery compartment door, the 27 should fit.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.