The 6" is a match for the old Viking hatch hole pattern. The 8" requires 2 extra screws.
Get the hatch "retro kits". That includes mounting gaskets and the screws needed on the 8".
71701001 HATCH 6" TWIST-N-SEAL RETRO
Hatch and gasket kit
Retro fit for Viking "T" handle hatches
71702001 HATCH 8" TWIST-N-SEAL RETRO
Hatch, Gasket and (2) screws
Retrofit for Viking "T" handle hatches
Serial number decoding:
http://www.hobiecat.com/support/articles/archive/?src=http://2010.archive.hobiecat.com/support/warranty/sailserial.html