Originally badly welded rear frame cross members tend to rip off the frame, especially if you kick start your Lambretta or sit on it while it’s sitting on it’s stand – which is also (only) fixed to the rear strut.
Series 3 Lambrettas: The inner holes are 106mm away from the center of the frame tube, the outer holes are another 123,5mm further away.
According to Innocenti original drawings, center-to-center dimension of the long-holes from front to rear strut is 184mm +/-1mm.
On Series2 frames the position of the front strut is different, it’s about 30mm further towards the front. Sorry, no dimensions here, have a search in the LCGB forums.
The upper face of the strut is 7mm above the frame tube’s center, according to Innocenti drawings. With a mounted rear floorboard holder you can get a line for the correct height by using a long straight edge from that floorboard holder to the front strut and clamp the rear strut to it.