As I recall:
There was someone on here a LONG time ago (2002 or so) who simply chopped the top off his half cab K5 and welded the gaps shut with small chunks of steel. They cleaned all the welds up and it looked pretty darn good.
AFAIK, you could do this too. The BIG advanatage of grafting on the old windshield section is the ability to put the full Fiberglass top on.
If you do not want to put a hardtop on, just cut away my friend! If you want a hardtop, then I think your best bet (BY FAR) is to go with grafting on the other windshield frame.
Other thing: The '73s to '75s had 'rocker boxes' that stiffened up the body so that it wouldn't sag too much when the top was off. If you chop your top off, you really should consider putting in a cage to keep your body from sagging and binding the doors.
Hope this helps!