I ordered the zerex HD ELC. They said their warehouse didn't have the concentrate and only had one case(6 gallons) of mixed. I guess I will add an extra gallon of distilled water and go with it. I picked up some test strips to make sure the extra gallon doesn't throw the temp rating off to far.
The problem with using mix, is you only get about 4 gallons out when draining your coolant system, so adding 50/50 mix results in a very low concentration. You'll want to do a good coolant flush to get all the old crap out and clean out the system. I recommend using Fleetguard Restore and then Restore Plus. (Restore Plus is an acidic cleaner like Motorcraft VC-9, but cheaper, and Restore is an alkaline cleaner you use first to remove silicate sludge.) Following these and several flushes of distilled water, you then add 3.5 gallons concentrate and top off with distilled. Even if you drain both block plugs and the radiator, you'll still have 1.5-2 gallons of water left in the system, which will yield only about 35-40% concentration if you use pre-mixed. If NAPA can't find you the concentrate (which they should be able to) you can use CAT ELC or Fleetguard ELC.
Concentrate part number: ZRX ZXED1 $17.99
Pre-mix part number: ZRX ZXEDRU1 $13.49
You can see the concentrate is also a much better value.