As big as that 75 is, I'd go with the "Joe's pasture reference." As far as markings on firewalls (in general, but not the one in the picture), they would be insignificant to someone whose truck does no still have them. If mine still had them, I would be stoked!
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission