Headers: you absolutely NEED custom made headers, unless you want to ding the drivers side in 4 to 6 places to clear the floor boards, the steering shaft, and the frame rails, and possibly ding the passenger side in at least one place. I bought custom headers from Dan Lemons for my tall deck engine. They can be used even with the factory steering shaft, without having to "dimple" them anywhere, and you can go as big as 2 1/8" primary tubes while still keeping all for tubes inside of the frame rails on both the passenger side and drivers side, and still have excellent spark plug access, (IF you choose his "Touring" headers).And BTW, Dan is a great guy to work with, and he won't even need to kid nap your car for 6 weeks like some other header places will to make custom headers. After a short phone call, Dan will know exactly how to fab your headers.
Oil Pan: AFAIK, the Milodon 31188 pan, and the Moroso 21047/21049 are the only ones that will fit a GM A-body frame, without any modification if you're lucky, or with a little bit, and still provide enough clearance for the crankshaft that you'll need.
Steering linkage: Good luck. You're gonna need it. If you're lucky, the factory linkage will clear the oil pan. If not, then you can either "dimple" the pan in two places where the inner tie rod ends will hit, or you'll have to go with a rack & pinion set up.
Hood clearance: IDK how tall your "L-88" hood is, (BTW, I've never even heard of an "L-88" hood for a Chevelle, so you guys have me lost on that one) but if it's at least as tall as the factory cowl induction 70-72 Chevelle hood is, (which is about 2" taller than the rest of the factory Chevelle hoods) then you have a good chance to make this work. Even with an extra 2" hood clearance, this will be tough, but not impossible IF you choose an intake manifold that isn't too tall. I have found that you'll have two choices of intake manifolds... The Merlin-X single plane manifold,(has a 6.23"/6.43" height) (#063041 for Dominator carbs, and #063031 for 4150 carbs) and the discontinued Edelbrock #2911 Victor TD, (which has a 6.23" height). Anything with more than a 6.5" height is going to give you problems with the air cleaner to hood clearance, and you can forget about using a carb spacer.