A swap I'm considering as well, Here are a few thoughts on it in no specific order:
An aod is going to have trouble dealing with a 351C. A c-4 beefed up would be a safer bet or a tko600.
Special headers/oilpan are indeed needed, I bought the moroso pan for mine, was about 300 bucks. Have not located a set of headers I like yet.
full length subframe connectors and torque box re-enforcing a minimum. Shock tower beef up would be good too.
Jury is still out on k-members, I'm not a fan of giant hood scoops so if clearance is that much of an issue I'll get a lowering k member.
Brake upgrades would be good, 351C is heavier than a 302 and a lot more then a 2.3. Jury is still out on a good spring rate.
Better radiator for sure. There are a couple companies making reverse rotation 351C water pumps so I will be going serpentine. Have not decided on if power steering is going to stay yet.
Rear end will need help, better axles, c clip eliminators, gears, probably a brake upgrade to disc.
4V heads have 2 piece valves stock that love to come apart if you rev it at all. As 351C love to rev, and make their best power reving, this seems a no brainer to correct. Good one piece stainless steel valves, bronze guides, whatever seat work is needed and a good valve job minimum. Have them machined for stud mount rockers as well.
Make sure your engine has the brass restrictor plate installed under the thermostat or it will run hot.
Your 89 will need its fuel tanked sumped for a carb application unless it is a trunk car and you plan on going fuel cell.
Plan on a large tire bill!!
I don't remember the stock number but I think a 351C was over 300 to the wheels ootb. Minimal mods will shoot you well past 400, mainly good lifters/cam/spring combo and enough exhaust to let it breathe. I have a 750 holley on mine but I won't know how that is until I fire it off.
Stroker kits for them are easily available, a 4V 408 set up well will give more than 500 of both hp and torque NA, in a 3000 lb car, thats a scary ride, hope the roads are dry and sand free!
my random thoughts
Matt