My 2000 9.6 2500 has a 3in vent that the battery actually sits on..Crazy.... Got to vent those hydrogen gasses somewhere... My battery box is only tall enough for 2 group 24's. 27's nor 6'ers will fit. Let me tell you what a pain in the butt it was to pull those batteries out of there to check electrolyte and measure SG...... I gave up on FWC batteries years ago.. I now run with 1 Lifeline AGM... I installed it 7 years ago, never pulled it out and rarely even open the access door. It still produces spec AH and voltage, honest!
Jim