It's normal for a manufacturer to source "the same part" from multiple suppliers. Many suppliers simply can't produce enough so they have multiple. As they received a batch in from X maker they used them, then moved on to a batch from Y makers, then Z maker. The vehicles likely rolled off the line in strings with heads all from 1 manf., then another, etc. There is no way for you to know by looking at the VIN, year, packages/options, etc.
IMHO, the amount you should be concerned depends on year.
Not all heads with "706" cast in them are Castech heads. Not all of them are troublesome but its a roll of the dice. GM claims to have had the issue almost entirely resolved by the '06 model year. I think it was a progressive thing where they used less & less of the problem heads. So the later it was produced the better chance you have of not having Castech heads.
IE, my '05 was one of the last '05 models to roll off the line. I have 706 heads and I will simply just monitor for problems. While it's on my mind, it's not of grave concern to me.
So I would say don't discount one with "706 heads" but carefully weight the risk VS reward. IE, if you find a 02 that's borderline good deal you may consider that the risk of Castech heads is higher, so it may not be as good of deal. OTOH, if you find an 06 that is a really good deal, the risk of Castech heads is lower and if there is a problem down the road the deal was good enough to defray some of the repair cost.
