Having the F engine and not the 2f in your 72, I would go with the 32/36. From what I have been told the 38 is too much carb. for the F engine..................
That said, I personally would just send off your stock carb. and have it rebuilt by either Mark's Offroad or Jim C., it will cost you less than a weber conversion and you will be more pleased than with a weber. Either of those guys can tweak a stock carb. to flow just about as much as a Weber . I know because I went with a weber when I fitst got my cruiser and now I have switched back to the stock set-up
Once I tweaked my Weber is has always worked - it does like Warm weather - not cold - needs very clean gas and air - wants properly tuned valves - needs lower fuel pressure- and it does not like steep angles- Which is why it sucks ass for a real cruiser.
I seem to get around 12-14 mpg with a 38/38 ,header, and 3.70 gears with 33" tires. I have yet to have problem with it off road, but most of the stuff around here is just trail riding, not much serious off camber stuff. But for a stock rig, you might want to consider getting the stock one rebuilt. Especially seeing as how weber has been on strike for quite a while and it doesn't seem like anyone has any carbs.
I have a non-USA Aisin carb on mine along with a non-USA dizzy and love it. Plenty of power and reasonable mileage considering a LC is like pushing a barn door around. The non-USA carbs are well priced and well worth the $$. SOR has them