Forgive me for asking the same question again, but I think maybe I didn't do it well last time, or I am not understanding the issue.
If you are using Billet Speedworks caps, or an equivalent pan rail to pan rail cap, why not use any 4-bolt motor mount block? You'll be cutting back the bosses anyway unless you plan to narrow the caps, or narrow them and use spacers as well.
Some say locating off the pan rail is weaker, but depending on component weight, a 2 bolt main can swing 600 HP, 6000 rpm. The caps are located off the register not the cross bolt, so you are really only grounding harmonics and adding another plane of stability, so they really shouldn't be any twisting force. If this is going to be a bigger machine than that, I''d think the rest of the block is going to call uncle anyway
Seems like I'd pick almost any good 390 4-bolt block, make sure registers fit nice and tight on the parting line, light fit with the pan rail requiring maybe a little lube to slide in by hand for reference, and make the block choice and machining easy
Thoughts?