1974 Series 3 SWB rebuild - rear crossmember replacement conundrum

I am in the process of replacing the rear cross member on my 1974 LHD SWB series 3.
The original cross member was extensively corroded and very badly twisted consequently there was no point in welding up a jig using it as a reference for aligning the replacement. Even so I did take various reference dimensions between various tub mounting brackets - I am also cross referencing dimensions form the rear body tub mounting points and dimensions from chassis drawings..

I proceeded to cut out the worst of the corrosion from the chassis and also cut the new cross member extensions to allow a but joint at the top and a 1 cm overlap elsewhere (or so I thought).

To align the cross member as shown in the photos below I have proceeded as follows:

1. The top of the cross member is set 30mm below the top of the flat section of the main chassis members/rails as measured from the least twisted sections of the original cross member (this also appears to tie in with dimensions from the rear body tub give or take a few mm ).
2. I have set the vertical face of the back of the cross member at right angles to the top of the chassis
3. The above alignment results in a 40mm gap between the bottom of the end of the extension to the bottom of the main chassis as seen in the photos
4. I assume the back face of the crossmember should actually be at right angles to a line drawn across the top of the three tub mounting brackets or whatever they are called. When I correct the alignment of the cross member accordingly the 40mm gap increases further.

It is evident from the photos that the extensions have a much shallower slope than the original. This could account for about 20 to 25mm of the 40mm discrepancy.

For those who are not yet comatose, I see three possibilities:

1. Ineptitude / ignorance on my part
2. poorly made / aligned extensions
3. I have bought the wrong cross member (I ordered DA4017 from LR Parts - this is for series 2a and 3 SWB so I assume this is not the case).

Any advice before starting to chop things about any further would be greatly appreciated - also is my assumption at 4 correct ?

Could you put the rear tub back on and see what holes line up?
Then if the rear cross member lines up with the crossmember to tub brackets use a few fixings and tack a few welds & re measure to see how far out things are.

Hi Hicap,
You are right - I will drop the tub back on and check again, Thanks.

I replaced mine with all the bodywork left in place. Then I could bolt it to the tub and tack it in. I only hinged the tub up about a foot to complete the welds.