The long 6 volt batteries are still made for some commercial uses such as tractors, trucks, etc. I got 10 years out of an Interstate from 1999 to last week in a 1948 Special. Exide sells one on sale for about $96 plus tax, F3EH is their number. Also available from NAPA, BAT7205 is their number.
I always keep the connections clean and tight, use only distillled water as needed, and always put on the Battery-Tender after each run. Of course you need to use the correct gauge of battery cables for 6 volt systems.