When installing my gearbox, it threw up an interesting positional problem which I'll reference below.
I have a very early S1, 238G (Jan '77), it seems that all of the S1 before approx 350G had slightly different features, and hence, go together away from what the manual says.
The problem I had with the gearbox concerns its position in the car relative to the other components, most especially the engine mounts.
The vertical aluminium uprights on the gearbox are slightly shorter (top horizontal dimension) than a late S1 or early S2.
I have the new hanger kit from SJ as the old bushes were shot, these are OK, however, when you move the gearbox backwards, the bushes eventually hit the upright bolt head, then you have to space the distance.
I'll list my calcs below, I measured from the face of the gearbox/engine block to the centre of the engine mount was 320mm +2mm -0mm.
[color=#FF0000][u][b]I have used 9/16 spacers.[/b][/u][/color]
This is how I have set my car up, rear suspension fits OK.
I will not have the final confirmation until I try with my stripped down engine block, however, this looks accurate.
For a quick check, for people using the Citroen gearbox, count the webs on top of the gearbox where they meet the rear chassis cross over. You should have between 4 & 5 webs showing, any less or any more and you're going to be out of position.