welcome too the web site!
yes the pre-1968 blocks tend to have slightly thicker castings,
if chevy could save lets say 15 lbs of cast iron through the use of increasingly more precise casting technology per block,
on lets say 150-200k cars and trucks, thats well over 1000 tons of cast iron ,
they need not purchase and machine.
how much thicker Im not sure, as casting technology improved less metal was used,
most factory SBC blocks are rated to hold less than 400 hp, ,
and yes many will eventually fail if constantly pushed to stress levels exceeding 400 hp,
yes we all know guys that built 500 hp sbc engines using a O.E.M. block,
theres a huge difference between having an engines total accumulated stress, and can claim bragging rights,
that may produce 500 peak hp on a dyno run or infrequent drag strip trips
and some guy racing dirt track, every Saturday night,
who keeps the engine spinning 5000 rpm-7000 rpm most of the time.
yes you can push the limits but stress is cumulative and eventually parts will fail if seriously stressed.
but any decent machine shop can sonic test your block casting before its machined,
the single rear main seal blocks are even thinner
the DART blocks come in several configs, all use a higher grade alloy casting ,
thats considerably thicker than the stock O.E.M chevy production block.
if you intend to exceed about 450 hp and about 6000 rpm with a SBC on a regular basis,
Id suggest use of a DART block and ARP fasteners,
and SCAT 7/16" connecting rods and a forged crank
http://garage.grumpysperformance.com/index.php?threads/engine-block-cylinder-wall-thickness.976/
http://garage.grumpysperformance.com/index.php?threads/types-of-crankshaft-steel.204/
http://garage.grumpysperformance.co...tion-of-crank-durring-short-blk-assembly.852/
http://garage.grumpysperformance.com/index.php?threads/what-con-rods-would-you-buy.942/
http://garage.grumpysperformance.com/index.php?threads/scat-cranks-related-info.10930/
http://garage.grumpysperformance.co...ds-vs-aftermarket-4340-connecting-rods.13321/
http://garage.grumpysperformance.com/index.php?threads/oil-system-mods-that-help.2187/