We use a Lawson utility called CNVEXP to create a fixed rec length direct deposit file.
It lets you convert files to fixed length records of a user specified length. See the System Utils Reference guide.
I've never tried it with a 6000 byte length though.
I wondered about 6000 maybe pushing the envelope. We actually use it for our ACH tape file, but the reclen is only 94:
cnvexp -f ...\pr160\1\tape 94 > ...\pr160\1\tapeout.txt