Would all the brackets (alternator, power steering pump, and so on) from an inline six-cylinder 258ci engine will work on an AMC 304?
The accessory drive brackets are going to be very different between the AMC 258s and AMC V-8s. Among other things, the engines have different mounting profiles. If you’re considering a V-8 swap in your CJ, then you’re going to want to source all the accessory drive brackets with the donor engine. Just as with Chevy V-8s, there was a variety of accessory positions and configurations on AMC vehicles among all the different car and truck models, so if possible get all of the bracketry from the same engine.
It’s worth noting that while AMC I-6 and V-8 accessory drive systems are not interchangeable, the accessories within the I-6 and AMC 2.5L engines are largely interchangeable. This means you can use things like the power steering brackets from a four-cylinder 2.5L on a 4.2L in a CJ, and you could theoretically put a 4.0L serpentine system on and earlier I-6 or I-4 engine (though there would be some issues with the intake manifold to sort out). This applies to the AMC 2.5L engine only; the GM Iron Duke 2.5L is a different animal.