I read all the posts, I do know the situation, and put that in my post.
Given the car is just restored, the possibility to damage it, by putting both assemblies in as one seems like the last choice I'd do, having done it once myself in 1974.
I also had a 69 GTO that was freshly painted. If you look at my signature pics it's the black flamed 69 GTO behind the 69 GP race car. I needed to pull the engine and transmission out of for work on both units, it also had headers. I was a nervous wreck reinstalling them as one unit, so I wouldn't do it again. Been there done that, not doing it again. I was younger (20) and less experienced in 1974 when I did mine, but it did get done, and the car was unscathed.
Yes it's possible, it also is the easiest to cause damage to a very nicely restored car too. I hope the OP gets it in unscathed. I'm sure there will be some that disagree, and that's fine, I have my experience's and they have theirs's. Everyone has the freedom to choose their operations on their car.
So yes it's possible, I did it, and so have others on here. Having done it once on a nice car, I wouldn't advise doing it. The engineers made the units separate and show in service manuals how they intended them to be serviced as separate units, that was the point of the post. Sorry if you don't agree with the points, even though I believe they are valid.
And the final thing is, I put in my
at the end of my post, although you may deem it worthless, and that's fine with me.