hi
dont wanna **** on your party but whoever told you that talking crap..
only way to make more power is to pend ££..end of.
so assuming you gotta a 100k + miles 3.9 then its running 100hp (ish)tops..due to cam being worn..
plugs will not add power..k and n will also add none..as if you would notice 3hp anyaway??.cold air intake might add aee bit..and its free..
but if i was you i would do a cam and lifter change..atleast then it will be back to stock hp..(and a full service, which by the way will cost the same as a cam)
to get power you need this..
but this does cost..but it works, unlike spark plugs etc..