Do NOT use steel caps
Why ?
Street motor with around 500 HP that will never see the track or 6200 RPM's
Steel expands at a different rate than cast grey iron does, same thing on aluminum main caps, the expansion rate and holes in them do not expand the same as the cast iron does
Hence my preference of using cast ductile iron
I had three RB blocks with cracked main webs
The 400 blocks are way better to start with than any OEM 440 block for any build that you want to make more than 600 HP on pump gas or above 650 HP on race gas
You had 3 crack where others have had none,something going on there.
Your correct sir, if you build enough HP or abuse the RB blocks enough they will crack sooner than the 400 blocks will.
I cracked one 426 M.W. RB block when a rod broke from spinning a rod bearing in low gear me and keeping my right foot down until it broke instead of lifting
The other two 440 blocks where cracked when the customers brought them into my shop
BTW, many other racers and engine builders have had the main webbing crack on many stock RB blocks
You can patch them up but you can't prevent the failures in the main webbing by bandaiding the stock RB blocks