Not a A-body but a couple of questions about a 318.

I'm gonna weigh in here just cause I'm bored. The LD4B is a great intake for the 318 because it has smaller runners that work well with the smaller displacement. They are not that common and can bring pretty good money if they are in good shape. That being said, if it's been hogged out to work on a 340 than it's basically not a LD4B anymore and you could probably get a Performer for less money. I would use this as a negotiating point with the seller, maybe you can get a few extra bucks off of the price.

I would not run a 340 cam in your 318. It's thirty year old technology designed for a larger engine that has bigger valves, higher compression and is made to turn higher rpm's. Buy a RV cam as recommended before that makes it's power off idle and down low in the rpm range. Get the matching springs. A timing chain is great insurance that the engine will continue to perform for years to come.

For what you say you're going to do with it, I'd stick some dual exhaust on it with the stock manifolds. It's a lot cheaper than headers and you don't have to worry about all of the headaches that come along with them.

A sure grip unit with some 3.55 gears would really wake your truck up. Limited slip differentials should be stock in all two wheel drive pickups. They are not very common but can be bought new.

Just one old guy's thoughts. Good luck!