Hear me out on this. Consider swapping to a 5.9 (360) Magnum instead of rebuilding your 318. For one the Magnums can still be found in good running condition fairly easily, and a 100k mile motor can just be freshened up with new rings and bearings without having to send the block to the machine shop. The motor in my car is just that, a 1xx,000 mile junkyard motor I freshened up 5 years ago and it's still going strong.
Another good point is that the 5.9 magnum will get pretty darn close to 400hp with just a decent cam, not to mention the nice torque increase due to the larger displacement. The combo would be simple. The basic combo would be a stock longblock with a decent cam (something similar to Comp's xe274hr), Crosswind dual plane intake, 650-750 cfm carb, and headers that would yield you a very nice running (still with good driving manners) small block that I would put money on outrunning that 318 you read about.
Really the only thing you'll be modifying is the heads. The only machining you'll need done is to cut the valve guides down on the heads to accommodate the higher can lift, but that is cheap. It cost $50 last time I checked to have it done. Other than that just install the recommended valve springs to match the cam and you'll be set. The stock rockers can be reused, as well as the stock roller lifters as long as the roller still turns smoothly and has no damage to the roller.
Making the swap into your car would be easy. The LA style timing cover and accessories bolt right up and you can run LA headers designed for your car on the magnum heads. I'm not sure on the oil pan but it might use the stock one from your 318, or one from a LA 360. As for mating the engine to the trans, the stock magnum flexplate will work as long as it's the one from the later magnums that had the weighted flexplates. If your motor doesn't come with one I've got a weighted flexplate you can have.
I'm typing this on my phone so it probably sounds all jumbled up but it's something you might want to consider. The total cost would probably be about the same or less than what you're planning, and would be a better upgrade IMO.
I'm available if you have any more questions.
