An inline six block tends to wiggle like a fish. If you left the two pairs unjoined you will be creating a stress riser by virtue of asking the block to bear the brunt of this wiggling motion instead having the head to stiffen the long block. I'm guessing the bores in cyls #3 and #4 wouldnt stay round long and block failure would soon follow, depending, of course, on the power level being developed. Then again, if you aren't looking for huge power increases in the first place there is no need for a hybrid head.
