Think of the 4.0L as the next generation 258...a lot is the same and will interchange, but there are a few differences.  The 4.0 has a larger bore with shorter stroke than the 258.  The trick is to get a 4.0L core engine, and stroke it with a 258 crank and rods, ends up being 4.5 or 4.7L.  Google "4.0L stroker" or maybe "HESCO Jeep", its all over the internet.

If you just want to do the head swap, that won't change the compression (much), but it will give you a freer flowing head.  Once again, hit google... "4.0L head swap".  There are some coolant passages in the head that need to be plugged, or put your faith in a thicker head gasket.  You'll also need the 4.0L exhaust manifold/header.  I think the 258 intake can be re-used.

The 4.0 was based on the 4.2(258). I don't know for sure if the heads are interchangable. The 4.0L engine has a 3.411" stroke, 3.875" bore, 6 cylinders, and a displacement of 241.5ci (3956cc). You can increase the displacement of your 4.0L engine by installing a stroker kit. At the heart of the kit is the 3.895" stroke crankshaft that was used in the AMC/Jeep 258ci (4.2L) I6 engine.On an engine with the stock 3.875" bore, the stroker crank increases displacement to 276ci (4517cc or 4.5L). Add 30 thousandths or 60 thousandths overbored pistons, and displacement is increased to 280ci (4587cc or 4.6L) and 284ci (4657cc or 4.7L) respectively. If you're feeling really brave, you can overbore your engine to 4.00" and use custom forged pistons. The block must be sonic-tested to ensure that there's adequate cylinder wall thickness before overboring this far, though. Displacement with the 3.895" stroker crank is increased to 294ci (4812cc or 4.8L). The next step up the ladder includes an offset-ground 3.98" stroker crank that increases displacement to 300.1ci (4917cc or 4.9L). The ultimate stroker includes an offset-ground 4.06" stroker crank producing a displacement of 306.1ci (5016cc or 5.0L).