Sometimes you can find a spec sheet for your proposed battery...
I usually keep a copy around for the Trojan batteries and if you can match up the battery weights the specs will be around the same.
TROJAN SPECIFICATION GUIDEBatteries in parallel you can combine their AH's but DC Voltage stays the same...
Batteries in series you add the DC voltage but the AH's stays the same...
In order to increase the AH's and DC voltage of the 6 VOLT batteries you have to have two parallel groups of 6V batteries in series...
I suspect your GP27 12V Trojans are around 100AH capacity - try to match them up from the chart above...
The 6V batteries with the very large core give alot of grunt power... Check from the chart how long the batteries will operate drawing certain amps load.
Roy Ken