Hi Oldbloke. A properly set-up Holley 390 on the Edelbrock Performer manifold is a joy, much more responsive throttle than the SUs, but thirstier. The new found ability to take off encouraged me to drive in a more aggressive fashion, which soon empties the petrol tank! The good old 390 gets maligned, but I loved it when I put one on my old automatic '73 3500. I prefer the look of the dull gold colour on the older ones to the shiny new Edelbrock/Weber 500 carbs too. Your kneck of the woods there must surely be a local Holley wizard who can tune it to perfection!? Use the extra power plates (I think they're called?) for extra tuneability.
If you do swap from SUs to a four barrel and you have no use for your old parts, I am looking for a better example of the big rubber gasket that seals the US export air canister to the underside of the bonnet!