I strongly believe in G.M. engines. Take care of it and it will take care of you. My 91 Chev Astro Cargo has 375,000 miles on it and still rins great. Original motor, tranny and rear end. Only repairs have been oil pump and timing chain last April just because I thought I should. Tired? Yes. Dependable? Yes Yes Yes! I put on about 25-30 grand a year.
I think cars and trucks run longer up here in the north because we are forced to take care of them. If we don't keep them in tune, they will not start for us when it'* 20 below 0. Our cars usually rust out on us long before the drivetrains die.
I'm not surprised. I have seen many Chevy trucks go well over 500k. In fact, here is a post from a while back (don't post in it ) about my great uncle'* Chevy that hit 600k in April '07. Not sure what it has on it now, but it still runs.