Don't know about opening up the passages, but I suspect the way gaskets are made anymore, it's not likely possible. If you could open up the holes in the gasket without weakening it, probably very little problem, but I also suspect GM made the cooling as efficient as possible, meaning the holes are already as large as possible for strength or gasket sealing, or are strategically designed to increase velocity or turbulence in certain areas.
One thing I've done on the last couple of motors I've assembled, is "port" the water jackets with a carbide burr, at least as much as I can see and reach.
Newer castings are much better (my Vortecs were extremely "clean") but a pair of 70's GM castings I had were absolutely horrid inside the water jackets. I could break off large chunks with a chisel and hammer, then smoothed them for flow.
I guess without some sort of before/after test its impossible to know whether porting the water jackets works well or not, but it stands to reason that less turbulent coolant movement means better cooling. As bad as the early castings were, I KNOW it helped move coolant easier.
