The heads make all the difference on the flow for a 340. Also compression. The 72 340 heads are only 1.94" I think it is. Earlier, are 2.02" You could resort to aftermarket heads but then cost comes in. Opt for 360 heads and get them worked.
Here's some info that may be helpful. This is a 318 mentioned mind you but the engines are cousins.
The "X" heads and the other 2.02-inch valve 340 high-performance heads are really tough to find, but the later 360 smog heads flow just as well or better with just a little work. These are easy to find, and also came on factory four-barrel 318s. Just have them machined for 2.02-inch intake valves. With a minor bowl clean-up, they will be as good or better than the 340 heads. The problem with all the 340/360 heads on a 318 is the chamber measures about 70 cc, which will drop the compression to an unworkable low ratio on a 318. You can forget about decent performance with these heads and the stock pistons, so again, a set of moderately priced aftermarket pistons is the answer.
I'll spell out the most cost effective and simplest combination. Start with the new domed Keith Black 318 pistons, which have a net dome volume of 6.2 cc. Drop these hypereutectic pistons in your block, and you'll have about 10.5:1 compression with a set of 360 or 340 heads, without having to spend the money for milling the heads or decking the block. The short dome is perfect for this combo. Just grab a set of 360 smog heads, have them rebuilt, machined for 2.02 intake valves (including a 75-degree bottom cut), and finish with some mild bowl work. Toss that Torker intake in the garbage (it's junk), and get an RPM AirGap intake and a 750-cfm carb. Use a .904-inch tappet hydraulic cam (Comp, Hughes, Lunati, or Engle) with between 224- and 230-degrees duration at .050-inch on the intake side, and a lobe separation no wider than 110 degrees. Put this combo together with headers, and you will be a 318 legend.
