apparently they're rated to withstand around 350 horsepower but are generally known to actually start blowing up around the 300 horsepower mark.
The (original?) trans in my '88 K1500 5.7L popped at ~230K; planetaries shredded, clutches cooked. Dead-stock TBI engine, nowhere close to 300 horsepower.
20 years later, (but only about 80K miles) the planetaries and the 2--4 band had had enough.
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When I was in the dealership in '84, emissions-choked V-6s and carb'ed V8s were tearing-up 700s as fast as we could rebuild 'em. There were incremental improvements over time, and a big upgrade in--I think--mid-year '87.
OTOH, my '03 Trailblazer has a ~275 hp DOHC six-popper, the 4L60E trans is original with a few fluid/filter changes, and still going strong at 280K miles and 21 years. The thing is old enough to vote and buy cigarettes. It does have an issue finding reverse when the temperature drops below freezing, first thing in the morning. Been doing that for about a decade, now.
did GM ever make a good head for the 350, or are they all junk?
Define "junk".
Nothing is perfect, especially in high-volume, cost-cut production vehicles.
I'm out $300 on Vortec heads at this point because 90% of the existing ones are already cracked or doomed to do so.
Did I miss that yours were tested and they found cracks? If they haven't been tested, it might be a good idea to do so before giving-up on them.
I've HEARD from everyone and his brother that Vortec heads crack like cheap drinking glasses...my favorite automotive machinist says he's never seen a cracked Vortec head. He has said that the Vortec heads that come on GM vehicles have head-gasket surfaces that are machined better than the Vortec heads that get sold over-the-counter at the GM Dealership. He says "I don't know how I would set-up my surface grinder to produce that bad of a surface." But that's his major complaint--improper machining, not cracks.
Aftermarket heads [NEW] cost as much as a complete [USED] LS engine.
FIFY. And hope that the LS you get doesn't have the heads made by Castech. GM has a bulletin on them...
www.gmfullsize.com
#06-06-01-019B: Information on Gradual Coolant Loss Over Time with No Evidence of Leak Found - (Jun 12, 2007)
every other head I've looked into seems to be considered only slightly less crack-prone than the Vortecs.
I think you're getting bad advice.
most of those parts were made of paper and broken dreams.
That's the nature of mass-produced items where cost is a concern. Nothing new there.