In an earlier post about the B230 buildup, you indicated a maximum of 120HP using twin HS6 carbs. That is a contradiction to the only tuning manual I've got which has a graph like the one on the SWEM website showing twin 1.75 inch carbs capable of 160HP. I know you are an experienced builder and racer so I reckon you've bumped into an horsepower wall on a twin HS6 setup. I got to wondering why that would be;

It depends on the manifold used. On a normal B18B manifold, the front carburettor cannot supply the rear cylinders or vice versa, however, if you build a tunnel ram type manifold,(imagine a pair of SU's bolted to a fuel injection manifold) then both SU's can supply one cylinder at a time.

The other thing is the figures that people quote. I tend to quote DIN nett BHP figures, because that's what represents the real world. What are they quoting? I'll bet SAE Gross, which is pretty meaningless. Much actual preference is MPH at the drags, that tells you how strong an engine is more than any HP figures.

I quote 170HP for my best effort for a B20 because that's what my Morosos slide rule indicates & my heads inlet only flow enough air for 185HP.

the carbs will move at least 250CFM each.

You say a HS6 flows 250CFM, at what depression? 1.5"Hg or 3"Hg.

The firing order separates the pulses so we get an uneven pull on the carbs. To overcome this, Volvo (and several others) utilize a log style manifold, and that is the source of problem. The very design that smooths the vacuum signal also destroys any ram effect.

There's no way to properly capture the tunnel ram effect that fuel injection manifolds & one throat/cylinder carburettors can do, the SU's piston gets in the road of that. You can capture a little inertia ramming using some ram tubes, but that's an icky area because only one of the cylinders of each pair is going to receive the effect which makes the mixture distribution problem worse.

I've done plenty of engine work on old Minis which have a similar problem, 99% of the best manifolds for Minis are single SU rather than twin because they fuel distribution is much fairer + inertia ramming is much more effective on a single SU engine.