The 6500/275 and 6500/300 shipped with 7.6.1 and likely won't support 7.5.5 or 7.6. It doesn't seem strange to me at all that the 6500 won't boot 7.6.

Several Macs are in this category including the Mach5 8600 & 9600 and the 2400c.

Peace,

Drew