Maybe i'll fix the transmission crossmember then put a wheel dolly under each tyre then remove the main dolly and bolt up.
That sounds about right. Maybe make two of the posts in your design removeable. It seems you could unscrew two top pins and then remove the base supports for thse 2 posts. Then you could support as noted above, pull two posts, insert two bolts and then remove the rig and insert 2 more bolts. That way your rig maintains alignment until 2 bolts are in place.
Tom