I had the same issue with my 88" 1963 short wheel base.
The lean started when I changed the rear cross member with extended chassis sections .. Measured /cut and re-welded as per job. Leaned after🤔
I have cured my lean by in un-doing the welds bottom and sides leaving top welded. On offside rear chassis.. Putting jack between axele and bump stop... Jacking up until chassis spring mounting are half inch closer together than other side... Then re-weld.. Squared mine up nicely.... Hope this helps..
