[Mike Taylor 3601]

I was told the same thing years ago,about no counterweights causing problems.They were running chevettes here on dirt track, some of them were cutting counterweights off,I balanced several of them ,they never did run long.
I do know 4.750 stroke BBC will run faster w/ crank w/extra center counterweights,even though it's heavier, than crank w/o extra counterweight. All I can guess is that extra center counterweight must smooth out some of the harmonics and in doing so eliminates some twist/flex in crank,and picks up some power.
Some signs of harmonics are balancer and flywheel mounting areas will show signs walking,galded surfaces, also where main caps meet block,or in your case main bed, will show signs of walking ,main bed may crack like yours before it walks,because of it being held in line w/alignment dowels.
Something else come to mind is a engine that is lugged will break mains/main webbing,Art you have'nt been lugging down the neon so you don't go too quick have you?LOL

Mike Taylor 3601