Dont forget the rear seat belt mountings in the floor pan as well, they will need a reinforcement plate underneath them too
Also there the side pieces attach to the vertical sections of the bulkhead you may need some right angled pieces to attach the two sections, if you have some sheet you should be able to fabricate some sections where needed
I did a rear body crossmember on my mates, his was so far gone it was easier to replace the whole thing with one from a donor vehicle, complete with the chassis mounts, I reinforced the mounts before fitting, and then it was a case of alignment to the vehicle and setting the door hinge side upright at the correct angle so the door would close properly at the end, bit fiddly with jacks and wooden props but got there in the end