340 Cam Kits which ones work with a stock block

Well, changing a converter isn't a track thing only. A higher stall just allows the engine to get into it's power making band a lot quicker and easier. I have a 2400 in my wifes car right now. It runs and drives like a normal car with the upgraded parts smoothly. With the 3.91's, you have a higher ceiling on what can be done. but a converter change would be needed for the bigger camshafts. Without a converter change, your limited. I would suggest no larger than a 218 @ .050 duration cam.
(Generally speaking, that is in the area of 1500/1800 - 5000/5500 rpm. Depending on how made the camshaft.)

That, "Nice sounding cam" sound often considered "That Muscle car sound" which is a cam ground on a 110 LCA. Most cams are done this way. A more muscle sounding, or more radical idle will have the LCA @ a 108.

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Would taking the heads off to modify them for valve springs be OK?
This maybe a needed evil and a lot depends on the spring used.