I have the "opportunity" to upgrade the main caps on a 340 block I was hoping to build, pending sonic checks. Opportunity comes from the main caps not being original to the motor, so they will need to be bored/honed to fit and they are also 318 caps as far as I can tell so i might as well put a "stronger" cap on while I'm doing it. My build goal is 550+ hp with a 4" crank and I don't think I feel comfortable with the supposedly inferior 318 cap which is supposed to be not as thick. So the question is, what material to use? The ductile caps a FABO member was selling are out of stock so I will be making my own. I've heard of folks using aluminum on big blocks, as well as alloy steels of various types. I can't recall hearing of them being used on a SBM and the only commercial caps I've seen referenced are ductile cast iron. I can get ductile cast iron as well, but it will probably be more expensive for the material than 4140 steel or 7075 aluminum. Regular grey iron probably isn't appropriate but I haven't verified the strength numbers on it. I don't think my HP goal is out of line for any of the three materials so what do I choose? Aluminum is easy to machine. Ductile iron would have same thermal properties as the block, but does that matter for something that will be street driven during the week and road course stuff every few weekends? Steel is the strongest. I tend to overthink these types of things. Depending on how complicated it is to make, I may end up using the stock rear cap and I've been told that is acceptable. Looking for whatever insight the group can provide.