My 68 400 is bottoming out and scraping bottom of mufflers and I am assuming that my 4 leaf springs are worn out and need to be replaced. I have found what appear to be decent replacement springs on Summit ($95 each), Classic ($125 each) and National Parts ($135 each) and I want to ensure that I am looking at the best performing lead springs. What lb rating should I be looking for - do I need HD or Regular? Also, should I also get new shackles, pads, etc. if I am replacing springs?