Group numbers indicate the dimensions of the battery case. It will vary some between manufacturers. But as a general rule...
Group 27 L x W x H is 12" x 6.63" x 9.06"
Group 29 is 13" x 6.80" x 10"
I'm guessing the tray your 27 is in has .17" wiggle room in width, but measure it to be sure. And make sure you have an extra inch of clearance for the 29 being taller. Hope this helps you.
"Things turn out best for those who make the best of the way things turn out" - Zachary Troy Schrah - a young man with vision far beyond his years.