According to Ak Miller and the details on the new shell moulding processes at the Cleveland plant, after 1969, Ford started downgrading the iron thickness. For early heads, 90 thou is the practical limit, but you can go to 120 thou like
FalconSedanDelivery did on both his head casting, and the block deck. Rocker geometry then has to be checked and adjusted.
After 1969, the average casting droped to 187 thou bluenprint at the Cleveland foundary, and for cylinder bores, they shell moulded down to 130 thou. Great if your making them, not so good if your reconditioning. Compared to a Chevy 283 or early small journal 327, the heads measure 242 thou everywhere,
with no less than 180 thou in the working face of the cylinder bore. Ford used to make engines like that, Y blocks, I blocks, FE's. Lima MEL 430 and 462's.....it wasn't a cost cutting measure made until Ford won LeMans, and then it came time to pay the rent. Less iron less cost. Its not an issue unless you really need a lot of compression.
60 thou for head planing for C9 heads to E1 heads, and 30 thou for overboring blocks. The 250 always had a lot of deck height, you can cut em down 120 thou, but block cracks around the head studs can occur unless some stress reliving is done.