i don't know if this helps, but american chopper, they build the tank and then lay it on top of the frame. perhaps you can cut out the bottom of the tank, so it slips onto the frame, rather than being assembled with the frame running through the tank? i agree, it seems like an annoying way of assembling it. another option is to cut the frame so that you can still paint the tank and add it on?
i suggest adding a picture or two to show the parts/assembly.