4.0 head swap benefits?

The head swap is apparently popular and a number of them have been done, but as in any modification the individual has to look at potential benefits balanced against the budget that you have available and what it is that you want accomplished.
The cylinder head gives the capability of developing more usable power and with that a certain efficiency in operation.
The cost includes a new head, the costs of modifying it to be used on an earlier block and new manifolding. That is intake and exhaust. And then finding one that will work with what ever fuel mixing system you wish to use.
If you can do it with in your budget and take advantage of the change in some way or another, it is probably worth it but only you can take that into consideration.
Personally I have built 3 engines since I knew it was possible and frankly can not cost justify taking a 4.0 head and realizing any particular benefit from it.
Purely from a cost to benefits evaluation.
That does not mean that I have not spent some incredibly large amounts of money for one thing or another, but I felt that what ever I spent it on was worth it to me.
Not to some one else.
But if the sole goal is to rebuild a 4.0 head and install it and not take advantage of it, doing so any improvement probably is not worth the money spent.
I am building a low budget performance oriented 232 at the moment and I have everything I want to use purchased at swap meets for next to nothing and have no intention of changing the head. If I did my shelf full of parts would be come useless.

Think its be more beneficial to just aquire a d eventualy regasket and drop in a 4.0 with 2wd trans while you continue to enjoy your 232 in the meantime

As stated, the 4.0L head doesn't add much by itself -- may 3-5 hp. If you want a header and nice valve cover you can get a 4.0L head/header/VC for about the same cost as a new header. This is especially true if there is a pull-a-part type yard in your area. You will need an intake as well though. 1980-89 AMC car/Eagle/Jeep Wrangler intakes will fit. The EFI intake can be made to work with an adapter -- either leave the injectors and rail on or plug the injector bungs. Some Jeep racers use the intake with a carb and adapter.  I'd just get a 2V intake from an 80+ engine and an Autolite 2100/2150 (1.08 version) since you're looking for a power upgrade.

With the early 232 you just have to be careful about pushrod length with the 4.0L head. It needs shorter pushrods. Someone on here has done this swap, can't recall what pushrod length was used though.

You will find a better performance increase by swapping the cam to something like an Isky 256 Supercam. The head you have should be fine. I'd put my money on the cam change first, exhaust second (2" single with turbo muffler is plenty!), then a 2V carb and intake (if you have a 1V now). If you can afford the intake, carb, and 4.0L head with VC and header, that wouldn't be a bad idea. Unless you find a 64-80 original 2V intake you'll need to change the intake and exhaust anyway. Any of the aluminum intakes can be easily modified to fit the 4.0L head -- factory 2V or aftermarket 4V.

You'll notice about a 20 hp increase with the cam alone (Isky 256 Supercam or similar). A Comp cam is fine, but stay away from the Xtreme cams -- they are mainly racing cams with very high lift, and require high pressure springs. If you like the split duration (intake and exhaust are different) call Comp and get them to custom grind one with under 0.50" lift. That will only cut down on high rpm power a bit, lowering the rpm range a few hundred rpm. You'd never notice on the street, and can use stock valve springs. The Isky cam is a great one like it is though.

I'd leave the header off, just use a 2" exhaust. A header will set you back a bit, as only Clifford makes one for the 232/258 that will work as is in a car. Jeep headers hang down too low. All the headers made for 72 and later cars will interfere with the left side starter of the early 232. They require the #5 and #6 header tubes to be cut and moved away from the starter. I don't think the current Clifford two piece header (two three pipe headers, for single exhaust you have a 2-into-1 pipe made) interferes with the left side starter, but I'm not sure.

As John says -- if you don't get the full instructions and all parts with the Howell you're in for a long ride installing! TomJ used one for a while and has a great write-up on installing that will help tremendously -- http://worldpowersystems.com/AMC/1963-Rambler-Classic-550-Cross-Country/Howell-TBI/index.html. 

As John pointed out, you can get just as much power from a carb as EFI. Where EFI shines is there are no more 3-4K mile tuneups. You can run 2-3 years without a tuneup, and when you do it's really just change points and check the cap/rotor/wires (if you have them -- many modern cars don't). EFI basically tunes the car as you drive, so you have more power potential at any given time. Carbs are great but can't compete! So under ideal conditions EFI and a carb will give the same power. Get very far out of the range that the carb was tuned in and EFI will produce more power and better gas mileage. Change altitude, weather, and even gas quality/octane and EFI will adjust engine parameters accordingly, a carb can't. A carb has some compensation built in, but it can't cope with very many changes. Carbs typically run a little rich as that's safer than running a bit lean. Too lean can cause burned valves and even pistons, to rich just makes gas mileage and sometimes power go down, and gets the oil dirty a bit faster -- EFI has allowed oil changes to stretch also. So a carbs efficiency varies over rpm (three circuits -- low speed, cruise, and high speed -- effiency drops at each end of each circuit) and with  environmental conditions, EFI adjusts itself.