Sorry, I was counting backwards 153624 and 135642 are the same depending on the crank direction.
Yes, you are right that the lack of crank overlap and small main and journal diameters create torsional windup. There is no good fix for this other than a new crank and deep billet block with larger diameters. This doesn't help at all for what you are doing now, so let's look at what can help.
Here we are working with a standard block split at the main centerline. A modern block with cross bolted mains is torsionally stiffer due to the deeper section, not the cross bolts. There are two ways to use some of this architecture on your block. The first is easy and reasonably effective, the second is a bitch and very effective:
1. Go back to the main caps and add a girdle. This is a milled part that is placed between the main caps and their nuts (longer studs are required). The girdle is a web that connects the main caps torsionally to each other through sleeve dowels concentric to the studs. It's a lot of work and it works best if you make your own main caps that are much taller than stock. Good main caps should have the nut face ~1 main diameter above the parting line compared to the Chevy standard of ~.3.
2. Mill a billet pan and sleeve every bolt to the pan. This makes the block deeper by a large margin and REALLY stiffens it up. This cannot be done with simple bolts, the sleeves are needed to transfer the shear loads that are created by the torsion.
You are right about the 1.75" header. For that piston displacement, we really need 2.25" to 2.5". In a high-boost engine, the only thing that matters is blowdown: the time from EO to pressure equalization. There is little scavenging above 20 lb and even less at 40+. Lack of restriction during blowdown requires diameter, so that each header pipe becomes a plenum.
I don't know what you can do given the owner's direction for this season, I hope this helps your long-term plans.
