it's logic...a cam,carburetor and headers just won't add 125hp to a small block.
perhaps 50 hp.
i'm betting most 408 strokers with stock heads won't hit 375hp.
..but i've been wrong before
Your basically listed all the main things that make power besides CR and heads and speaking of which 340 has both maybe not the 10.5:1 advertised. If you go by the 2hp per cfm rule of thumb X heads have 440 hp potential. So yes a cam, carb/intake and headers can add a lot of power. Question does this combo, I think it's maybe a little generous at 390hp, to me a 400 hp combo is 10:1, 280 ish cam, 750/air gap, 220 cfm heads and headers. And that combo is two steps down on cam and a half cr down so should be 30-40 hp less not 10 less but even if there dyno is a little happy still useful info.
And for us not in the racing world probably we get our sense of engine hp is from magazine builds and even if it's a little askew it's actually probably more of the norm since a greater number get our info that way. And really there's so many variables it hard to get an accurate hp number of your engine anyways even the run it down the track method.
And of course the 408 gonna make 15 hp less (375 vs 390) cause it eats it up in friction. lol