I want to change out the 40 year old studs in my 71/454 el camino; the ones i find at the auto parts store are abut 3/16" shorter than originals. What is the normal procedure now? Should I drill out the rotors and go with 1/2" studs instead of the original 7/16" ones? Any other suggestions?

as long as you can still get full thread engagement in the lug nut with the shorter studs, they will work fine.
otherwise you need to get the parts jockey behind the counter to walk away from the computer and get the book off the shelf with the wheel studs in it and find one that has the proper knurl diameter and the same length as your stockers.
or you could take a look here- www.dormanproducts.com -to find the part number you need and then go order it at the parts store.

Go to your chevy dealer and get them, ones I got are the same length as original.