To drill the holes properly,bolt the pump to the adapter,bolt the adapter to the block.Take a drill that fits snug in the upper hole in the pump,about 17/64 or so,drill just enough to leave a starting point.Then use the 5/16 drill keeping it centered in the pump hole as best as possible.
I just finisher my 4th adapter job,nice if the holes are drilled perfectly,but very close works out just fine.I just drill right through into the water passage,tap it all the way through,use sealer on the bolts.On some block,the 3 bottom holes on the adapter will have to be enlongated just a touch to screw in the bolts in without forcing them.
Remember,you can easily remove just the pulley from the pump by drilling out the 4 spot welds,no need to press off the hub.
On some of my trucks,I still repositioned the radiator side brackets to gain an extra 1/2 of room,making it easier to remove the fan and or pump without pulling the radiator.
I like the paddle blade stainless steel flex fans,they pull a lot more air at low speeds and the fan tips can be "clearanced" carefully with a snips to clear the damper if necessary.You do need to enlarge the center hole a little to fit,as the 216 pump hub is larger in diameter.Or just press on a 55 or later hub.
