One thing most people are missing about the 401-534 series and its limited performance is the poor combustion chamber that the "planked" head gives relative to a better quench-style.
The surface to volume ratio of this style of engine is off the chart in the wrong direction, even though there is functionally no chamber to speak of.
If there is an upside, you can sneak a pretty big valve in there and the only problem is hitting the bore itself. Problem is that it stops breathing about 6000 (not a problem in a truck, techincally) because the incoming charge hits the cylinder wall instead of tumbling, swirling or dispersing itself.
The heat does everything but work in this engine, there is soooo much area when the piston comes up to the top of the stroke. Chevy 348-409-427 (early) Truck motors and the MEL Fords had this same configuration, and all were discontinued for pretty much the same reason and replaced by nearly identical engines (BBC/385 Ford)
The Ford exhaust port dogleg is almost always a function of having to put the engine in a chassis or package it was never designed to accomodate. The 429 got its port bent to fit the Torino/Mustang unit body; ditto the 335-series.
And the genesis of the GAA was an aircraft, not a cramped Sherman tank, so I'm sure that if the engine had been produced in its original concept, the output numbers would have been dramatically higher with a cleaner raised exhaust port, forced induction and eventually fuel injection for inverted mounting or acrobatic flight. All the liquid cooled multi-cylinder 12's had that by '42-3 including Allisons and Merlins.