Some of Millions' gang show up, looking to collect. Do you send them away empty-handed and risk tankly response? Do you pay up? Do they notice your antitank preparations? Are you sure? Are you willing to murder some kids to be 100% certain you take that tank unawares?
Are any of the PC's gang NPCs? Do they really want to go up against a tank? Seriously, boss? This is a good plan and all, but... tank. Inch-thick steel, shells the side of my head. Well, Rolfball's head.
Millions, meanwhile, needs gas, parts, and ammunition (if he ever gets around to shooting anything). Where/who do they come from? Do those people talk to/are the other PCs? Maybe they don't come from anywhere, and that tank is toast... shortly. Millions has just long enough to piss a bunch of people off, and then it's just him and a bunch of gutter rats against, er, everyone Millions extorted/shot at coming after him. What do the PCs do if he comes to them for help? Or maybe a more sympathetic member of his gang, all of whom are on the chopping block thanks to that pint-sized megalomaniac?
If it does come to a fight, this seems like it might be a good time to break out the Battle Rules! Their plans and traps give a framework in which they maintain untenable positions (fighting a tank), provide covering fire, face an increasing number of bullets and shells as the clock goes to midnight, and eventually throw a hand grenade in to Millions' lap. Now they've got a busted tank on their front lawn. How busted? Who wants it? Is it the PCs? Well, now they need gas, parts, and ammunition...