xdef 540 linear -180 0.666672
ydef 361 linear -90 0.5
zdef 42 levels 1000 975 950 925 900 875 850 825
 800 775 750 725 700 650 600 550 500 450
 400 350 300 250 200 150 100 70 50 40
 30 20 10 7 5 4 3 2 1 0.7
 0.5 0.4 0.3 0.1
tdef 4 linear 00Z01JAN1979 360mn
vars 8

--- to make grib1 template ---

grid='255 0 540 361 -90000 180000 128 90000 179333 667 500 64'
copygb -g"$grid" -x in.grb out.grb

The grib1 has less precision in the lat-lon than grib2, so wgrib2 was
used to "poke" new values of lon1 and dlon.

changed center to 255:255
--------------------- to grib2 --------------

 wgrib2 -no_header 4rec -set_metadata metadata -import_bin z500.bin.dat -grib_out z500.grb2