70 318 with 340 internals/Cyl Head

It sounds like whoever built that motor thought a lot about what they were doing. As for the 318 340 crank they are the same part number for the forge crank however they are balanced different because of 318 has lighter rods than a 340 and the Pistons are lighter. You can put 340 rods in that 318 it'll change the balance so I would think whoever did this had the rotating assembly balanced but I could be wrong. There's too much money in the add-ons for them to have not done it right. You can say a forged 318 crank is a 340 crank if you want to but they are not balanced the same. As for the valves a 70, 318 did not have 1.88 intakes. It had something like 1.78 intakes and 1.50 ehaust and the early 340s had 2.02 intakes and 1.6 exhaust valves not 1.65.
It's bored .030, so the guy put money into the block why wouldn't he have bought good pistons. Run a compression test on it. See what that tells you. As for the torker 340 that is the early version of that manifold I believe they had torquer 340, torquer 1 and torquer 2. Somebody correct me if I'm wrong.