From the Leyland Parts Catalogue, 20 being the front header and 17 the rear
C-post trim with five clips (arrowed)
Rear header rail with four clips (arrowed)
An unused hole at each end, which are adequately supported by the C-post trim pieces
Detail of a rear corner
Suggested dimensions for the main board (however early cars seem to have the material stuck to the roof panel). Note that 'visible length' is with the rear header panel removed i.e. to the rear edge of the board. The visible width over the B-posts is also 38", it only tapers in to 37.75" in the last few inches before the join in the cant rail and front header rail covering.
Detail of a front corner. You may be able to print this out and blow it up to use as a template, but it is easy enough to make one using a card 20.75" x 10", sliding it into the gap, drawing round, and trimming the corner bit by bit. Given the amount of overlap it isn't that critical.
Vee's headlining - just a thick layer of compressed orange fluff with a layer of thin card between that and the vinyl covering. Very easy to crack if handled carelessly.