The timing belt on my parent's suburban broke while they were towing a boat. Yeah...that engine was toast. To be fair I'm sure it was past due for a change. I don't know what the recommended service interval for those chevy 350's was but I think they had at least 150k miles on the clock. So timing belts are really not that big of a deal but it really is important that they get changed at or soon after the recommended interval.
Toyota says 90k miles for the 4.7L V8 but you know they have some extra tolerance built in. I'm sure most of them can go 120k with no problem. But is that something you really want to risk? I'd rather pay to get it changed early than blow the engine.
Chevy's 350ci 5.7 has allways had a timing chain. Never a belt.