That's exactly like what I made, I also welded on some flat plate to prevent the bolts marking the pan rails

i just thought i'd pop in since we are talking about the main caps and girdles. the bcr/sdc caps are made from 6061 aluminum, saying the 7075 is heat treated doesnt mean much all aluminum is heat treated its just anothing buzz word to sell caps to an uninformed public. we have always used 6061 in those caps, 7075 has a particular flaw that lends itself to cracking more in cyclical loading applcations. yes its harder than 6061 but the grain structure also makes it more susceptible to cracks. we've done our best to eliminate the stress risers on the caps. (chamfered and radiused corners etc. but the main take away with the sdc/dcr caps is that they are significatly taller than the other stock replacement after market caps. we dont use washers or spacers as the cap directly touches the girdle and the steel acts like a big vise cinch everything together and help distribute the load. in my opinion, without that interaction, adding a girdle to a stock style cap doesnt add any strength and can actually create an additional stress risers at the main bolt interface as the center of the cap is not supported and the material flexes.

With all due respect, not sure I can accept the term "flaw" here.
All Alum fatigues over use.
My knowledge is much different than you have related.
I am and have always been suspicious of those who steer towards 6061 vs 7075 for similar "common sense" reasons you share but in reality, it's just maybe that 7075 is so darn expensive and harder to source.

"Key takeaways:
6061: Moderate strength, excellent corrosion resistance, good workability, cost-effective.

7075: High strength, superior fatigue resistance, less corrosion resistant, harder to work with, more expensive.

Key Difference: 7075 is 84% stronger but 20-40% pricier. Choose 7075 for maximum strength, 6061 for better workability and cost-effectiveness."

https://aluminummetalsupply.com/6061-vs-7075/

Got any sources to support your position, please share.

i'm not disputing the fact that 7075 is a stronger material. as was mentioned 2024 is also in the mix. we could also go an use a 5086 material, or why stop at the aluminums? we can use 4340 steel or if we need to go lighter weight can make titanium caps. there is plenty of relatively inexpensive titanium on the market these days coming from china. i dont know how good it is but its probably stronger than 7075. the main take away, you use the material that fits the application. in this case both 6061 and 7075 are stronger than the cast iron cap they are replacing and stronger than the cast iron of the main webbing. if we were seeing cracked caps in 6061 then we would have to switch to 7075 or something else. but in reality we aren't, none of these engines being put together are being built the same in a production environment, and it's impossible to get a baseline in terms of power uses and longevity with identical tunes etc. all of us are pushing the envelope. if a max effort design was being attempted and money was no object an aftermarket block probably makes sense, but then so would making titanium caps. we are trying to lessen the impact on the bottom main webbing of the block. for this to happen its more important to trace the stress path. with our girdle caps and even the stock replacement caps we make, the stress path is through the cap and into the girdle finally into the oil pan rail. not through the main webbing through the caps into the bolts and then maybe into a girdle and then the oil pan rail.

also this is the main reason not to use the cross bolted caps where you have to machine away the factory main webbing supports as this changes the stress path of the main block itself.

so in closing its more important to look at the overall design of the cap and how it fits the application more so that just the material type itself. there can always be a case made from look its stronger/harder/whatever, but that may not be the best for the application either.

btw we source our aluminum from the USA for these caps. and furthermore, we can make our caps out of 7075 with a custom order and will price them accordingly. we have made alot of custom application in the past and have even made caps from 5086 and an aluminum version of our girdle.

" if we were seeing cracked caps in 6061 then we would have to switch to 7075 "

That would pretty much answer the whole question right there.