If the mainframe is initiating the FTP, the command would be
quote locsite trailingblanks
Something else to consider is that Unix files are not record-oriented; the line termination character ends a text line. If you transfer this file to a mainframe file with RECFM=FB, and the line on the Unix server is shorter than the record length, it will be padded with spaces at the end to meet the fixed length record requirement. If this is the case, you either live with the trailing spaces or you change the mainframe file from RECFM=FB to RECFM=VB, adding 4 to the LRECL, so the records are picked up at the length they are on the Unix server.