Best manifold for ThermoQuad-equipped 340

The OE iron is a very good intake
There is a toss up between the LD-340 and the Weiand Stealth. I have not run the Stealth. I have run a LD-340 with a milled center divider and minor port work. VERY GOOD!

In the single plane world, the Holley Strip Dominator, then the MoPar M-1 which is a enhanced version from MoPar

Your looking for a street strip 12 second ride correct?

The OE intake is capable of doing so. You'll have an easier time with an aluminum unit though and the 2 dual planes should do it easy enough for you