All SBC's have "Siamesed" cylinders, not just 400s. To say that coolant doesn't flow between the center cylinders is pretty inaccurate. There are cooling jackets around all of the cylinders and around every port in the head, here's a cutaway showing that:
View attachment 249223
What "Siamesed" means WRT the SBC is that the center two cylinders have their exhaust ports next to each other (note the "red" ports in the cutaway are exhaust, blue are intake). People really make that out to be more of an issue than it is. Yes, it does create the possibility of a bit of a hot-spot in the very center of the head, but this has little/no impact on overall engine cooling. What it DOES impact is the potential for detonation in the center cylinders, but especially in carb'ed applications the intake manifold design tends to lead to those cylinders being a bit richer anyway. That's not to say SBCs don't have cooling issues at times, but it has nothing to do with the intake/exhaust port layout of the heads.
Also, looking at the OP's pics I see 8-lug axles (and what appears to be a full floater in the rear), that rules out a 10-bolt rear. I assume you meant 10-bolt, not 10.5". The 10-bolt had a 8.5" ring gear, even the 12-bolt was a 8.875" ring gear and wasn't a full floater. The Corporate 14 did have a 10.5" ring gear, but I've never heard anybody call it a "GM 10.5 inch" rear. That said, it obviously is 8 lug and full-floater, so maybe that's what you meant? Either way the OP said it has D60s front/rear - and the rear 60 was available full-float so that seems to be the case here.
As for the OP's carb issue, I'd convert it to EFI. It's just too easy these days, with kits that replace your carb with a throttle body and injectors on your current manifold and are self-learning/tuning. As others have said, IF the motor itself isn't healthy then an LS-swap might be a much better plan, but if the current 400 is happy I'd just swap in a Holley EFI setup and call it a day.
-TJ
