Tires not clears firewall headers

That's pretty common on "A" body big block with fender well headers. Original wheel was a 14". Like he said, smaller tire/wheel combo, or you can jockey with wheel offset. TTI or Schumaucher makes some nice "inside the frame" headers that will solve your problems. In the old days we would massage the header tubes just a little and change to a different torsion bar. Not to sound like a smart---, "A" bodies w/big blocks are made to go straight!