70 318?

Just take your free 318 "boat anchor" and turn it into a 400 hp street engine for the price of a rebuild and some out of the box magnum heads. The dyno may be generous, but hell, that ain't bad.

http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html

And there's nothing magic about the 340's. They were great engines from the factory because they had decent flowing heads, 10:1 compression and a hotter cam. Do the same thing to a 318 (as they did above) and you'll have a great engine that probably makes 20-30 hp less, but cost as much as 1k less to build if you can get it for free. BFD.

Using the above build, you can figure the 318 makes about 400/318 = 1.26 hp per cube. A similarly built 340 will make around 1.26x340 = 428 hp (maybe, the heads might not flow enough for that in stock trim). But even if it did, you'll pay at least 500 bucks and probably more for a 340 core, and the pistons cost more.

And unless you're on the track, you may not even notice the difference, since most of it will be at 6,000 rpm.