Did you have larger cables put on ypur Classic? As i seen the otjer one and no matter what battery yoi use it still won't start until you put the beast back together.
That beong said along time aho a awnry cus recommended a speed cell battery as a second battery. 3 regular batteries later and that speed cell is still kicking. That was 2010 and all I can say is dang Art you recommended a good battery. Not I run two batteries.
Motobatt all the way, but with ground cable and starter upgrade, probably any decent battery OK, I never had any cranking issues.
Edit: Posted before I read your post, I'd find the issue, do you have an alarm? Mine sits a month or more, still starts no problem, even when it had a 5 or 6 year old stock battery, something else is going on.
All I needed was the extra cable going to the ground. Cheap and quick fix. 1 battery turns over the 265 hp motor like it was nothing. Fast starts every time.