What the heck is NA ? Did you mean Nitrous ? but I still don't see how stock parts are holding up unless those are 340 stock rods and crank but cast pistons and 87 octane?
NA meaning Natrually Asperated. (Serious spelling errors today.)
Stock MoPar rods are a strong steel. More so than other brands of car manufactures. So are the blocks. They have a higher nickel content.
Cats pistons are seriously underated. You can get away with alot if done right. Things like a light car help short comings in many areas like small displacement, lower comp. ratio's, low stall and suspension not up to par.