Re: SB main bearing failure
No; I think the BLOCK wasn't cleaned up as thoroughly as it should have been.
I always pick up my block from the machine shop after they've been vatted and machined, but before cam bearings or ANY plugs have been put in; and I take it to the quarter car wash on a sunny day in the morning, along with a couple of cans of engine degreaser and a set of gun cleaning brushes. I run the rifle/piston/shotgun brushes through EVERY hole and passage to get rid of loose casting sand, debris from previous blow-ups that happened in the block, machining chips, etc. etc. etc., with plenty of degreaser to help disslodge anything lodged in oil build-up and varnish that might be left over; and let it sit out in the sun and dry all day. Then I paint it, put the cam bearings and plugs in, and oil the cyls, lifter bores, main saddles, etc. to prevent rust.
Coolant would have affected all 5 main bearings, and all 8 rods, equally, and would have made them ALL look like it was run without oil; heating, pitting, metal flaking off the bearings, etc. since antifreeze turns oil into something that doesn't lubricate. It doesn't produce gouges. A few bearings with deep scoring, and no generalized evidence in the rest of the motor of lack of lube, is evidence that the vertical oil feed passages to those particular mains, or the groove behind those cam bearings, had trash in them.