With proper prep and machining, I've seen 350 4-bolt blocks approach 1,000 hp. They didn't have to LIVE there, but they could touch it without exploding.
"Proper Prep" includes complete "hot" machining done at 200 degrees F, aftermarket billet steel main bearing caps, ARP main bearing studs, Hard Blok up to the freeze plugs, and top-quality components throughout (billet crank, rods, etc.)
The block should be magnafluxed and sonic tested prior to starting anything, and either cryo treated or vibration treated before machining. Doing both wouldn't hurt.
~SP~
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