4.0 head swap benefits?

Any engine can gain some from better breathing, but of course a larger one will gain more. The old 232/258 heads have large ports but they are pretty much a 90 degree dump over the valve, not very efficient at all. The 4.0L head has smaller but much better shaped ports, that's why they flow better/more.  Increasing the flow will increase power on the 232 without a doubt, but I do agree that the increase is better on a larger engine. Unless the 232 had been recently built on the bottom end or has relatively few miles on it, I'd go with a 4.0L or 258 as well. The only obvious exception is if you needed a pre 72 engine.

On a different take, the 232 will spin up just a bit faster than the longer stroke 258, less of a difference between the 232 and 4.0L. For a drag engine built for max rpm that might make a difference. I have an article from the early 60s where a guy took a 54-55 Nash Rambler (58-60 American size) and used a 172.6 crank in the 196 block. Block was bored 1/8" to get around 180 inches. Swept the field in his bracket class (sixes up to 200 inches, I think). The extra rpm did it. High rpm is not conducive to long engine life though, so not a good idea for a street car -- not with an old school engine. It's not one of the modern small fours that seem to last a lot longer even turning a lot more than our old school designs do. EFI and better oil and other technology helps...