I Am Considering A 4.0 Head On My 258 Engine, Which Year Of 4.0 Head Is The Easiest Conversion? Would It Help Any To Put Headers On This Head Or Is It A Waste Of Money? I Have A 1980 Cj5

The numbers cast on the head

you want a 7120 or 0630

I think that's basically years 91-99
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There is no thing as an easy conversion, not that it is difficult, but you need to pay double attention to detail.
Recipe recomends a HO head, wich you'll find in '91 and newer.
The 4.2 headers do not fit in the 4.0 head.

Not quite right.

After the 0630 heads they choked the exhaust ports down for emissions carap.

The factory 4.0 headers are actually acceptable flow wise (4.2 headers will not fit). The problems is they crack. There's some remanned headers that are welded up and have flex pipes installed to alleviate the cracking. Don't know if the cracking is fixed, but it's worth trying for $100.
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'94-'95 4.0L heads were the optimum choice for you. That's the one that I used. After 9 years on there, it's still good. No woe with the 4.0L header. Mine was from a '96 XJ. There are lots of details you need to pay special attention to. I used head bolt sleeves on all 4 corners. It's difficult to find aftermarket headbolts (or was 9 yrs ago) which you want to use. To fill those pesky water galleys that will be dead ends, I used, don't laugh, styrofoam peanuts and marine filler, kind of a heat resistant putty. Hesco gave me advice through the process. My engine (4.4L) is a .060" overbore, 258,4.0L head, with Cloyes 2xroller chains, advanced 4 degrees to lower the rpm it makes peak torque, Hesco cam, hi vol. oil pump, adjustable fuel pressure regulator, etc, etc, to make peak torque at lower rpm. It pulls great at 800 rpm in 130:1. I also have a 'no idle increase' anomoly with the MPI so I can dump it down to 4-500 rpm to smoothly grind over the rocks.
I will say that the thing I lost in building it specifically for low end torque, is the high end. On the highway it's a dog pulling steep hills in high range, compared to a stock 4.0L which has more high end power.

Mud toy,
noticing your tag, maybe it would be best to find a '94-95 YJ or XJ 4.0L engine and just transplant the whole thing. All in all, they're cheap, run forever, easy to find in a wreck, not that bad a swap, and....
will give you a better high end for your Mud Boggin.
regards, as always, jefe

If you have a 79 or earlier 4.2, there's no need for special headbolt stuff.
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