I'm not sure if you're talking about a header generated with HEADERx or a header from somewhere else. But if you want to create a file with LRECL=742 from a file with LRECL=502, you just have to ensure that the output records are 742 bytes long.
For example, you could pad the 502 byte records with blanks to 742 bytes with OVERLAY in INREC, OUTREC or OUTFIL, e.g.:
If that doesn't help, then please explain what you're trying to do in more detail. Examples would help.
Note that you will actually have to break the 742 character constant into multiple constants each less than or equal to 256 characters. And you will either have to make each constant fit on one line (e.g. use multiple constants each less than about 60 characters) or observe the rules for continuation of constants across multiple lines (specify characters up to column 71, then put an * in column 72, then continue in column 2 of the next line).
See the following for DFSORT's continuation rules: