Hello I am new to this forum as I am primarily a long time MOPAR fan. I recently purchased a 1969 SS396 (325 hp) and love it. Only had it a week and a half. It ran great when I first got it (it was restored by the way) after the initial dead battery problem was solved with a slow charge. I then took it to local well respected and trusted mechanic shop near me for a once over since I am not a mechanic. I got it back and was told it did not need much but they did replace spark plugs. The bill said 454 in the description for the plugs. I thought maybe they just typed it wrong as it is a small shop. When I drove it from their store to my storage garage, it stalled several times when I slowed to an idle. It did not do that when I first got it. A well informed friend came to look at the car today and said it sounded as if it was not hitting on all cylinders and instead of him adjusting the idle, I should take it back to the shop where It had been. I will do that Tuesday but wonder if you experts can tell me if there is effectively that much difference between 396 and 454 plugs. I looked for sizes on line and it appears most of the 396 plugs have longer threaded bases (at least in the pictures). Thanks for any other thoughts to throw out to the old time mechanic when I take it back Tuesday.