I guess you should build my motors.
I have seen a true, dyno results for rear wheel increase on a big block switching from matched manifolds to stubby headers to be about 12 hp increase. At the fly wheel that translates to roughly 16hp increase.. you will not get 30 hp increase from a set of headers and a muffler, especially on a small block magnum/mopar with a stock induction and heads. primarily because Magnum heads flow like crap.
to add, engines actually make more power as they age. As valves seat, rings slightly wear into the bore, bearings slightly wear you increase the efficiency of the motor and decrease the power loss to parasitic friction, the motors ability for it easily pass the air and fuel through basically improves.
Most motors will reach optimal efficiency around 80K miles, assuming the motor is kept well maintained and lubricated, that efficiency should stay steady for hundred thousand of miles.
