1st 340 build
340 cost a lot more to build and are hard to find. 318 and 360 easier to find but why build a 318 when you can build a 360 for the same money as a 318 but make more power with a 360
This isn't really inaccurate.
340's don't really cost any more to build than 318's or 360's, as long as you're talking about similar builds component-wise. The only extra cost is finding/buying a 340, which can be a big cost. If you already have one, then the rebuild price is pretty much the same.
Machining costs will be the same for any small block. Reconditioning the crank and rods is the same (or the same cost as a 318 if you buy new parts). Only real difference is the pistons. Usually I would say the 340 pistons would set you back a little more. But a quick check at Summit says that 318 pistons, KB 167's, cost 319.99. KB 243's, 340 pistons, cost 255. Go figure. So the pistons don't have to cost more. A 340 timing set, gaskets, bearings, oil pump, etc are all the same as a 318. Head gasket may be different depending on the bore, but it shouldn't be that different $ wise. Comparing complete rebuild kits, they're the same. I see Summit doesn't have a cast piston kit for the 340's, but they're out there (or you can sub in the KB hypereutectics). And the forged piston complete rebuild kit for a 340 is priced the same as a 318.
As far as rotating assemblies go, you would probably be better off buying a complete rotating assembly since you don't have anything right now. Buying used parts that need to be reconditioned might actually cost more money than some of the new set ups if you're not careful, depending on your local machine shop and how much you pay for the used parts. For example, if you pay a couple hundred bucks for a forged crank core, and another $100 or so to have it reconditioned, and you could have bought a brand new cast crankshaft.
Depending on your goals for the engine, a cast crankshaft can be more than sufficient, and quite a bit cheaper. If you're shooting for 500 hp stay away from cast, but for a stock to moderately built engine they're fine.