[Tiara Open] Need to replace 6 Gel Batteries. (3 house & 3 engine) - 38 Open w/450 Cummin's

[ReelMagic]

I'm getting ready to change all 6 GEL batteries as they are 18 years old and serviced me well. Never had an issue starting but this year when starting for the first time after winter layup I needed to bring all 6, via the parallel switch, to start.... In any case I looked to see what size they are but was unsuccessful. Know they are either group 27 or 31.... Anyone know which ones were used or can I use the group 27's to save approx $500.00.. CCA is lower (550 vs 505) as well as reserve power (190 vs 160). the size difference is small.... Suggestions greatly appreciated