4.0 head swap

So which of the 2100's - what CFM, did the VAM engine come with? Remember, for best results there's one that is much better than the others.

The issue with any I56 is the intake - unless one goes to a dual plane after-market type. 
What he'll loose with the 258 intake and 4.0 exhaust is the warm intake - depending on climate, it'll be bound to stumble and freeze the throttle plates. i have first-hand experience there I hate to say..... or admit. I've fixed that issue for others who went the route of mixing and matching intake and exhaust and ended up with a cold intake in the winter or fall or early spring. When the temps dip to about 40 or less and the air is very humid, like we have here, you can freeze throttle plates in place and end up with a car that won't slow down unless you push the clutch or put it in neutral and let it rev. I had my throttle freeze so many times it ticked me off and I went back to the stock combo with a properly working heat system for the intake. I had headers and a stock 2bbl intake - froze it solid, and there was frost on the bottom of the carb in cool moist weather. the 258 aluminum intake can resolve that issue due to the ceramic heater on the bottom that runs until the water temp reaches about 165 degrees then the warm water takes over and prevents carburetor icing. More velocity through the carb, worse problem. It's science  ;-)

The problem with a large carburetor is the lack of velocity and fuel suspension - and you lose low end torque too. Too small and you starve the engine for air/fuel and lose power, too. 

If I were to design an intake, i'd do like they do with the V8  - one side of the carb would feed the front cylinders, 1 side would feed 4, 5 and 6. No sharing - that's where you lose velocity as each cylinder takes a big gulp through a big double-throat. Or I would go variable venturi........

Wasn't it the smaller 2100 that was best - can't recall but there's a number on the carb to watch for. 

The 4.0 is a superior head. The comp split pattern cam can gain you incredible throttle response - many who have used it in the 4.0 or 258 can attest to that. I can mash mine and she GOES. "Snappy" is how I refer to it. In fact at times I do fear for that 19 rear end under my car. I can make things pop really hard. It will be a lot better once I can afford the Bosch 703 injectors. That's a proven dyno checked gain in power - and MPG as well. My injectors have about 120,000 miles on them, original, never been cleaned. It's snappy now, with remanufactured superior injectors she should really wake up. 

I do recommend the comp cam to take advantage of that 4.0 head. Don't put a 258 stock cam back in it. higher lift, better flow, that will take advantage of the ports in that head and if the intake is port matched very well at all it should do great. 

You can check for the 2100 carb and adapter kits - complete with instructions, etc. on eBay but MAYBE someone here could cook something up for you!

You CAN put a 258 (4.2L) head on a 4.0L block.... but WHY???? You have a poor flowing head on a newer model block. No advantage at all. Plus you would have to run an electric fuel pump -- the 4.0L block doesn't have provisions for a mechanical pump.

It's very easy to put a 258 intake on a 4.0L head. The "ears" on the bottom of the 258 intake that go on the alignment studs just need the bottoms filed a bit so that they sit on TOP of the studs. The top edge of the intake should sit even with the top edge of the intake ports side of the head. That will align the ports -- as good as a stock intake (4.0L or 4.2L) aligns anyway. The harder part is the washers to hold the intake to the head. The change in distance between the intake and exhaust manifolds mean you have to make plates or grind oversized washers to fit and clamp the intake and exhaust manifolds to the head. Not hard to do with an angle grinder, and preferably a vice.

You MUST use the 4.0L exhaust manifold (or aftermarket 4.0L header). When the intake ports were raised the center two exhaust ports were also separated. On the 258 they are siamesed into one port, so the old 258 exhaust manifold won't fit. The factory exhaust manifolds are really headers themselves, not much point in an aftermarket unless you can't find a good factory piece. Even the more log style RENIX exhaust manifold is much better than any 258 exhaust manifold.

As billd noted, the head alone doesn't add much... 5-7 hp (with the factory exhaust manifold). You need a better cam, intake, and carb to really get much of a power increase. You get around 5-7 hp for each item -- so 20-30 hp, depending on what parts you choose.

As far as what 4.0L head.... ANY 4.0L head outflows a 258 head. The 87-90 RENIX head is the worst, but even it is better than the old 258 head. The intake ports were raised on the RENIX head about 1/8" higher than the 258, the 91+ 4.0L H.O. head has the intake ports raised an additional 1/8" (1/4" over the 258). Raising the intake ports allowed a better curve to the intake ports, whereas on the 258 there is basically a hard 90 degree bend from port on the side to the valve.