Are you working on a 440 block or a 400 block?
The reason I'm asking is that the difference in materials from cast grey iron to steel would make me , and I don't, not use steel
I have used ductile iron main caps with great success on my 400 builds, same on aluminum caps from 440 Source, no steel though ever
The 440 block will and do crack the main webbings before breaking the main caps, not so on the 400 blocks I've seen
The steel material will transfer the harmonics into the block more so than either aluminum or ductile iron is what I have been told by several machinests
I have cracked one M.W. 426 block and seen several 440 blocks with crack main webbing with the stock caps on them in one piece