Drove the car home several hundred miles with the old carb, a gearbox that wouldn't shift into third and a sticky kickdown linkage. Gearbox and linkage now serviced, still chasing an overheating issue but that seems to be down to a faulty gauge. Goes right up to 100% within the first ten minutes before the (new) thermostat opens, then settles at around 70%. No actual signs of overheating though, checked with an infrared thermometer.
With the rebuilt carb I covered around fifty miles but not in one go.

That's a great way to ruin fuel mileage with the stock type choke.

Ok, time to wrap this up now:

initially planned to rebuild the engine when a friend offered me a '72 318 which he rebuilt himself. Swapped that in and since it already came with a 4-bbl Performer intake I put on a 500 cfm Edelbrock AVS.
Sent the radiator away to have it re-cored at the same time and ditched the factory a/c.

Had the 904 overhauled too but of course the linkage didn't fit the new carb, which again led to rather disappointing results. Ordered the Bouchillon Performance solution but for the time being the stock setup seems to be working okay after a few mods. Not perfect (yet) so I'm still undecided whether to stick with it or install the cable conversion.
Installed a new exhaust too. Couldn't fit duals but the larger diameter single exhaust seems fine. Hated the Flowmaster muffler though and swapped that for a Dynomax: much better!
Installed a cheapo AFR gauge/O2 sensor which is a bit hit and miss. It does show that the carb is jetted surprisingly lean @15.5 AFR for part throttle cruise.
Maybe I'd better check the spark plugs as well...

Fuel economy improved significantly but only when I replaced the gas cap did I finally see 21-22 mpg on a 50 mile test run in town.

Was it all worth it? Well, at around 5.50 per gallon it's eventually going to pay for itself and while the engine and carb swap wasn't strictly necessary the transmission and exhaust definitely were. Doing away with the non-functioning a/c also meant losing considerable weight up front and it cleaned the engine bay up nicely. You simply can't get R12 over here and I found the pricey Sanden compressor conversion in my other car slightly disappointing and hardly ever seem to use it.

temp gauge still doesn't work but I added these:

That open air filter doesn't help economy or power.
The engine now sucks in hot underhood air.

The original airfilter housing on your other engine is a much better design, feeding the engine with cooler/fresh air.

15.5 AFR during cruise is ok.
As long as that number goes down when you accelerate or put more load on the engine.

I would also install a vacuum-meter to monitor the condition of the engine and maybe tailor your driving habits.

Nice (economy) upgrade none-the-less!