If you look at the drawings(bottom drawings) on page 173....look at the sketch dimension drawing in the left upper ignoring the CAD photos...
this sketch shows the assembly for the NEARSIDE, it is basically like an Xray vision picture, you are looking down thru the top plate.
You are correct in saying the bolt holes are NOT central to the plate, as in the diagram....the forward holes are 45mm from the front leading edge ( that sits on top of SB2) & 20mm (this being the outer hole) in from the shorter edge of the top plate then 70mm in & likewise for the other 2 holes creating a 70mm box with all 4 holes.
So you are correct on the 45mm...70mm....35mm spacings & yes the 45mm gap is the one that sits over the SB2 rail.
I will say that the most important point of this assembly is getting the shock bracket in the correct position as this then lines up with the shock mounting point in the rear upright.
The 4 bolt holes are just for attaching the mounting plate for the roll bar .
1 more point to note........when you weld RS15 into position don,t forget to leave a 3mm gap along the back edge which is where CP6 is then welded into position ( this is the tiny dotted lines in the sketch that are not easy to see/notice at first glance ).
The 2 CAD photos at the bottom of page 173 are wrong as they imply that the shock bracket is positioned on the outside when turned over ......the shock brackets should be located towards the diff cage .......close to where D12 if that makes sense & the 4 bolt holes are positined towards the outer close to SB1,s if that makes sense
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Sorry for long winded reply
not easy to explain , hope it helps .
cheers
andy