Totally agree with Scotty on this... I had 232,000 miles on my GTP before I even started to mod it. Added headers and a 3.6-3.4, custom tuned, ran it hard occasionally and the engine was perfect. Sold the car when the transmission pooped at over 300K. Body was rusted bad so it wasn't worth replacing the transmission in my opinion.
Probably true, but I put SD (cheap, ~$200) headers on my Impala LS (same n/a motor) planning to top swap later... and it turned out to be a decent mod N/A. Not night/day like on the L67, but definitely some gains on top if you have decent ignition timing.your na? f it, leave it alone.