Your tank is already drilled? Sorry its a little hard understanding these confusing things online lol. Do oyu have any pics of your current setup? That would make it easy as 123 if we could see how it already is.
What i was saying was to have a typical setup but with a canister filter. (The canister isnt really neccessary, was just trying to work around what you wanted
). Also your going to have to make sure that the output of your canister filter and the output of your return pump are rated for the same gph. Otherwise youll see problems.