I just had my '67 '5 apart the other night- same issue- left rear was an inch down. I put all new Omix springs on it 2 years ago- 7 leaf front, 9 leaf rear (original was 5 + 9, but 7's were cheaper?) Removed a few leaves to soften it up, down to 6 on front, 7 on rear. It sat good for a year, then started sagging. To even it out, ended up adding 1 (the longest and most arched out of what I took off) back to the left rear to level it out.