The overall frame size for one of these i just found online is 964mm x 664mm. The opening is about 510 x 860, so length wise will have about 50mm each end sitting on the existing old bricks. Width would have about 70mm sitting on brick each side (one end is narrower and would be about 40 or 50mm sitting each side). Does that sound sufficient - is that what you meant by well supported? Thanks.