Since you already have a 258 I would go with the 4.0 head, plus you could do some other cool mods. Lots of goodies for the sixes. My son is building a 258/4.0 for his AMC Spirit - raising the CR, Clifford manifold, Holley 390, split headers, Lunati RV cam. He's using a 5-speed so I don't have much to contribute regarding the tranny, but the 200 and 700 GM 4-speed auto tranny's are possible too.
1977 Cherokee S