318 vs 340 Questions

I would trade 'em both for a good running 360. Those 20 cubes are hard to argue with,(cuz they represent about a bit better than one cam size),and allow a better cam selection before getting bottom-soft.. I just know I'll get flak for saying it but If you have to spend money on a rebuild You might as well get the most bang for your buck. I owned every size of SBM, and by far, the 360s are the most fun. Forget the cast-crank to steel stories you have heard. The 360, in a streeter, doesn't need to spin 7000Plus rpm. Although mine has for 16 years. A streetable cam in the sub 230* range, only needs to go around 6500 with a 4-spd to kill most comers. A 220*ish cam to give plenty of torque only needs to go 6000 to 6200.That 360 cast crank is one tough piece.

I did not like that 292/509 in a 360 Barracuda @ 3450 pounds(me included). The bottom end was pretty soft. My compression was 10.82Scr. It needed a starter gear of 10.5 to 11.5 for it to come off the line respectfully. It will be even softer in a 340, and especially so with a wider-ratio 904 behind it. Unless you put a pretty loose TC in it. And then it will be no fun at all on the hiway, cuz you are looking at 4.10s.Of course your car is a smidge lighter than mine; perhaps as light as 3300 pounds,car and driver. That's worth nearly a cam size, right there I guess.
Here's the thing; if you optimize the Scr for the 292/509 cam, and you find it not to your liking, it's very hard to back up the bus and fit a smaller cam in there cuz the engine may not run WOT on pumpgas with an earlier closing intake valve. If however you set it up with a smaller cam, and find it too small, A bigger cam can often be fitted with a thinner headgasket, and still run on pumpgas.
You may have heard it said that when presented with two camshafts, chose the smaller one. For street, this has been my experience.