Ok, I'm playing with an 81 Spirit with a 258, and have heard about the 4.0 head swap. What special pieces do I need, or is this truly a bolt-on? Will I need a different intake and header than one for the 258? I am going with a 4bbl and header, 3.54 gears, mostly a weekend bracket car (was gonna get a 401) but will drive it every now and then in town. Is this gonna be worth it, or am I better off to keep with what I have and work on the 258 head?

Get the '91 - '93 4.0 head as it flows the best - raised ports.

Yes it bolts on - with some minor mods - like epoxying closed some holes used during casting - that become exposed since the 258 is narrower than the 4.0 - no biggie.

Best intake would be an '87 - '88 AMC Eagle water cooled / heated Alloy Intake - use a Mr. Gasket 4bbl to 2bbl adapter - this is what the MOAB JEEP Race Engine builders use - AMC / Renault spent a pile of $$ on Design & flow.

258 head is a money pit - forget it.