Sounds like you are on the right track. Have you tried using 20 gauge cushon wad/shot cups?
Last year I bought and put together a kit from North Star West. It is a 62 caliber smooth bore early english trade gun. It seems to shoot tighter patterns when I use these shot cups. I shoot 80gr goex with equal volume of #6 shot with orange juce wax carton over shot card. It works pretty good for me but I am still experimenting.