Starting my parts gathering

The 273 Steel crank and rods are a good starting place but remember free generally means they will need gone over. In fact it's just about as cheap to buy new Eagle or SCAT I-beams as it is to get ARP bolts put in old rods and have them re-sized which I can almost guarantee you need to do to 43+ year old rods to stand up to 400 hp. KB Hyperutectic (sp?) pistons are fine for a 3.31 stroke. Their pretty cheap too. Just remember to have the rotating assembly balanced no matter what route you take because the 273 crank is balanced for the bob weight of a 273.

400 hp is doable on a "teen" but you will sacrifice drivability. I doubt your stock heads even with mild porting will make much more than 300 hp. You will need to step up there if 400 hp is your goal. A lower cost alternative set of heads that can make 400 hp on a 318 are the RHS heads. Brian @ IMM engines (member OU812) sells them mildly ported and set up to go with good beehive springs for about the same cost as out of the box Edelbrocks and from everything I've read they out perform the out of the box Edelbrocks. Don't get me wrong, the Eddy's are great heads once you correct the sloppy valve job and check and ream the valve guides so you don't hang a valve open. Their quality control needs to be better. Even with Eddy's you'd need to do some porting to get 400 hp. I believe with the right cam (solid preferably) the RHS heads can make 400 hp on a 318. But along with that you'll need to run a pretty loose converter and low gears or it'll be a pig out of the hole.

You said you didn't want to stroke it but you may want to set down and figure up the price to do it as a 318 vs. a stroker. I say that because you may be able to find a stroker kit just as cheap as it'll cost to build the 318 due to all the machining expense of re-using old parts. Another big advantage is a stroker makes so much more torque than a 318 it won't need nearly as much stall speed and low gears (can we say drivability). Plus it'll be much more docile than a 400 hp 318. Just a thought to consider.