Horsepower Gain with Custom Tune?
 

I posted something about this before but i think i may have worded it in a confusing way so ill try this. I am thinking of a dyno tune later on this spring. I heard on a bone stock '02 F150 5.4 2v you can gain from 20-40 hp and i dont know if thats at the wheels or just at the crank. I have a slight exhaust leak on the right side with the stock manifold and was considering some Gibson shorty headers to open the flow a little. I was thinking about a 50 Series (1 in, 2 out) to let the flow be a little better and get a quiet little rumble with it. I do have on right now the complete K&N Intake system. Was considering a Spacer but i hear bad things about them, but will they work with a custom tune? Would a bigger change anything? I guess that computer has to be able to read any mods done to get the gains. What im asking is does anyone know about my average HP will be with and without all these mods done? And this is with 93 octane tune or whatever the best one is. Is there a such thing as an E85 tune, which is 102 octane right? I did read that on a dyno tune there is more advantage for gains on a 2 valve than 3 valve Engine? I also considered a small shot system of ZEX nitrous instead of a tune but nitrous just scares me, i feel safer with just a tune. Any opinions or suggestions? If i have incredible gains (hopefully) will the engine, trans, and rear-end(3.55) withstand the added power?