Looks interesting now!
You know, now you have the gearbox out I will revert back to the original suggestion of using nice thick bits of aluminium plate as the adapter and aligning the motor to the gearbox by standing it all on end.
However, we now have a cradle.
Where you have cut the box section into channels it would be good if you can close the channel by welding the cut sections back in. The transition from the square section to the flatter rectangular section can be blended with a taper to reduce stress risers. Some reinforcement around the internal corners of the cuts will be needed to replace the lost strength and rigidity.
The bolted sections that don't need to be removed should be welded when their location is certain. The brace across the top will need some location lugs welded on to rigidly locate it prior to bolting. Once you have that in the right place clamp some bits of angle to the underside of each end of that brace and weld them to the brace. That will prevent any chance of the cradle closing up when the bolts slip, they will slip. It is easier to show you but I can't get down to you at the moment.
But that brace will move, and let the cradle sag, if you don't have some positive location on it. I'll have to give you a call at some point and we can talk about it.
I am still not convinced that 3mm aluminium plate will be able to transmit the loadings in the coupler. If you consider how hard the steel clutch center is compared to how soft the aluminium is and that the torque from your motor could damage the clutch centre....
I reckon you will shear the aluminium very easily.
The vacuum tank looks good as does the cable duct and cooling duct.
Do check for clearances between the brushes, brush conductors and the cooling duct bolts/rivets. It looks ok in the photo but hard to be sure.
I am still immobile. No driving, no workshop, no chainsawing logs, no stone wall building.
I spend all day on the couch or scooting about on an office chair to get around one room and I use crutches to get down to the granny flat for dinner once a day. Arch pushes me about in a wheel chair each weekend to get some air.
I have another hospital appointment on Thursday where I will find out if I can put any weight on my leg at all.
Likely four more weeks before physiotherapy to make me mobile again.
Must look into getting that Land Rover axle too.