need advice on what cam , springs and things to get for my 340

lemme show you something;
here are three cams of the same 235*@.050. To show their worth as to fuel economy, you have to work from the advertised numbers, so, say this 235 cam is 274/282 advertized.
I will show ALL the event durations so you can see what I'm talking about. The first will be on a 112LSA, then a 110, and finally a 108. To be fair, I'll put them all in at split overlap.
intake/compression/power/exhaust/o'lap/Ica/LSA
274/113/105/282/54/Ica of 67*/112LSA
274/115/107/282/58/Ica of 65*/110LSA
274/117/109/282/62/Ica of 63*/108LSA
Notice how the exhaust duration grows with each LSA change, from 105 to 107 then 109, two degrees per 2 degrees of LSA. By the time that the exhaust cycle gets to be less than 105, fuel economy has already been suffering. Out past ~116 is strictly for economy cams. So in the middle is about 110.
Next; notice that the overlap grows twice as fast. Overlap increases the operating range of the cam, and thus the power. 62* is a pretty good street number for a 360. But the 340, to make power,will want a lot more. Like 70/72, assuming cams from the same manufacturer..

Next lets take that 110LSA cam and change the cam timing on it 2* each way from split overlap,

274/117/105/282/58/Ica of 63/110LSA (plus 4*)
274/115/107/282/58/Ica of 65/110LSA (plus 2*)
274/113/109/282/58/Ica of 67/110LSA (plus 0*)
Now you can see, from the power-stroke range of 105* to 109*, that you can take this same cam from chitty fuel-economy at 105*, to merely mediocre at 109*.. If you had lots of compression ratio, you could retard the cam 2* and pick up another 2* of power extraction, but the cam is now losing Effective overlap, so the operating range may suffer.

Next, lets tighten up the duration split from 274/282 to 274/278.
274/115/107/282/58/Ica of 65/110LSA (plus 2*)
274/114/110/278/56/Ica of 66/110LSA (plus 1*)

lookit that, we picked up 3* of extraction, but lost some exhaust duration. No big deal with headers and a free-flowing exhaust. Good deal.

Next; lets go to a faster rate of lift, that has the same .050 numbers (and beyond), but starts out with shorter ramps. Lets knock off 3 degrees at each end, so now we have a 268/272/110 and the numbers;
274/115/107/282/58/Ica of 65/110LSA (plus 2)
268/117/113/272/50/Ica of 63/110LSA (plus 1)
Whoa, now yur talking potential for fuel economy.
Finally, lets put it on a 106LSA for gear splitting
274/115/107/282/58/Ica of 65/110LSA (plus 2)
268/121/117/272/58/Ica of 59/106LSA (plus 1)
Whoa, that's a lotta potential right there!

Lets give some extraction up by going to the next bigger cam.
274/115/107/282/58/Ica of 65/110LSA (plus 2)
268/121/117/272/58/Ica of 59/106LSA (plus 1)
274/118/115/278/65/Ica of 62/106LSA (plus 1)
Now yur talking; this cam has it all;
adequate compression degrees,
PLENTY of extraction
lots of advertised intake/exhaust
a great Ica to work with
a great 106LSA for the GearSplitter
a funtastic 65* of overlap
plus, hopefully, still is a [email protected]

Ok so there's some food for thought...........