3.6V should be fine, but that said, I've seen the Varta brand 3V ones for under 10 bucks a unit on ebay, whicch were OEM fitment on late 80's Apple's and Mac's such as the SE and IIgs so will even look mostly factory. To my understanding, the 3V ones are actually 3.6V anyway... Somebody correct me if I'm wrong.

But yes, if the Mac II won't boot, and hasnt been recapped, I would start there... I've never come across a Mac that early that would fail to boot due to flat batteries alone. Later Powermacs, particularly the TNT and Kansas machines can do some funny things with no PRAM battery, but even they will power on and boot... they just need a little persuasion.