In other words, to have a convertable 6:1/7:1 the upper (triple) block would need to have a becket. Add a standup block, mounted through the hardware that attaches the main sheet cleat, to your lower block. Your cleat is on the bottom, you add the extra block on top of the "plate" that supports the cleat. You're spending for an upper triple w/becket and one small block only.

Another option is the new "carbo" triple as the upper, and instead of buying with a becket, run the tail of the main sheet through the hole in the center of the blocks and dead end, in effect same as w/becket, but maybe a few more bucks.