CSP hit most of the high spots, here are some others....
If you have a cable clutch, your linkage adaptation is going to be simpler, although the cable system is somewhat unreliable ( be sure to keep it well lubed). Otherwise you will have to adapt the engine side mount.
If your running an HEI on the Buick, upgrading the primary ignition system would be beneficial.
Exhaust: fenderwell headers, in frame headers, or modified single pipe, in order of ease and cost.
The V6 will be set back from the radiator. This means either a long fan extension or electric fans. If you go electric, now would be a good time to install them.
On a more obtuse direction.
Along with your engine swap, I would seriously consider a transmission transplant as well. The T14 is very marginal at best, and I have seen many break behind the 225 Dauntless odd fire V6 (mine included). Perhaps an upgrade to a T150/176 or even one of the truck boxes would save money in the long run.
All that said, and my love of the Buick V6 engine aside, the 258 is an excellent engine. For me, it would be a hard justification to swap it out for the Buick powerplant. Not trying to dissuade you, just that it seems to me, in this case your swapping one good engine for another, which may equate into a lot of work for little gain. The 258 is highly upgradable, and will last a lifetime. It is also already mounted in the CJ. Perhaps a transmission upgrade, disk brakes, power steering, lift, traction diffs, etc. would be money better spent.