We have a Thermador GGCV30 gas cooktop. I would like the dimensions of this cooktop so that I can compare it with others which we ay choose to replace it. When I measure the Thermador, it appears to ave a depth of 19 1/2 inches. When I measure other so-called 30 inch gas cooktops, they have a depth of 21 inches. Are these interchangeable? I also have to allow 3 inches for a telescoping downdraft vent. We do not have enough clearance in the cabinetry to allow for anything wider.