[rotorwrench]

Modern fuel injection and electronic ignition all with computer control gets the economy about as good as it gets and same with performance. To put a swirl tube device in the plenum likely doesn't help the vaporization all that much. The amount is likely just above negligible due to the amount of blockage drag there is in there. (One might help a bit while the other affect detracts a bit). It's most likely affect on economy would have been slowing down the fuel mixture to a degree that may have given a slightly better burn. The way a flathead is made, there is already enough chaos going on inside the cylinder to create a good burn but with the low compression, it also loses out a bit on performance.

Now an actual turbocharger might get the performance up a bit but then the economy would likely go down a bit too. It's always a trade off somewhere with these old flat motors.