For the '65-'66 with the smaller instrument cluster this is a good way to go.  The Rally Pack puts the gauges in a spot that's considerably harder to see.  I've always mounted the tach right in my line of sight in my race cars.  Its so much easier to see, which is why Shelby put it there. 

Gauge wise I've always used Autometer and with very limited exception been satisfied.  I've built custom gauge clusters for four different vehicles over the years and always used their gauges.  I will say the higher end gauges tend to be nicer, though with the exception of a tach that died early I've never had an issue with accuracy or longevity from any of their products. 

If the dash is original I probably wouldn't drill holes in it, especially if its in good shape.  If its a repro I'd drill away.  I've drilled, cut, modified, etc. a bunch of repro parts for my car while keeping the originals.  Likely they will never wind up back on the car, but I feel better about not having destroyed them.