I've seen lots of drag cars with custom fabbed tanks alot smaller than stock. I know that my '83 tank is rediculously huge. I don't need 60 dollars worth of gas at a time. Anyways, it's no big deal to mount up something different because it's just held up there with straps anyway. You probably just would want to keep the guauge lined up with the acess hole in the back for easy acess. In a Z it's even easier with the external fuel pump. You could even drop the stock tank, steam it, and cut an inch or two off the bottom, and weld a peice of sheetmetal to the bottom, trim to fit, and make a little sump with baffles around it.