There were no 4.0 heads available in my area, Farmington.
A local Jeep hot shot who owns a wrecking yard here makes sure of that.
So I bought a new ready-to-go ported and assembled head from Hesco.
Clifford has a similar offering.
The heads from these two outfits have already had their sometimes troublesome manifold side outboard triangular water jackets welded up and have been re-decked.
If I were to do the work on a used one I'd probably get the jackets welded or at least JB weld them, then take my Foredom flex shaft tool out and at least clean up the ports and combustion chamber, and have possablly have new bronze valve guides installed.
Others have upgraded and slightly enlarge the valve sizes, though this somtimes isn't a really good idea due to the higher temps caused by off-road use. The larger, thinner tulips of these hi-perf valves will run hotter, compounding heat build up. Plus this mod gains little advantage in off-road situations.
Your 40K mileage head will probably be OK as-is.
I'd first try a trial fit to see if you are going to have a jacket problem ...
this, I have learned, depends a lot on the particular year of your 258 block.
Many get by with no mods in this area. Others have had success using JB weld on them, then taking a large mill bastard file to the globs.
Going to 1/2" head bolts (if necessary) is always a good idea.
Machining them ($50. locally) is always the best route, but I have had one report of a sucessful hand tapping of the stock 7/16" holes of those model years (risky).
Jay at Clifford has assured this board recently that they will sell their adapter sleeves if needed.
Other cheap touch ups can include a good rotary wire brushing of any carbon build up and possably hand lapping the exhisting valve seats.
This is the old-fashioned way ... using a suction cup on a dowell stuck to the valve face to spin (lap) the valve while in it's seat using a fine abrasive silicon carbide grit.
In any case, if you dis-assemble, do keep the valves to their respective ports.
Keep your lifters and pushrods to their respective valve assembly unit also.
A thin re-decking at this point is also a good idea.
If you are talking about going through the entire engine, the list of mods is posted on my site.. which include 110/100 ft.lb. torqe plate installed boring, Badger pistons, ARP rod bolts, double roller chain, heavy duty oil pump and complete balancing .. including the dampener, flywheel and clutch.
I could have poured even more dough into it by installing roller rockers, forged pistons, and hardened or forged rods ..... $$$$$