Does he still have the floor pan? Seems to me some frame braces and a 340 would have been a lot easier. Hope he has better luck with his BMW parts than I did with my 2010 328i convertible. It was a very unreliable car. I sold it just before it went out of warranty. Loved to drive it. Didn’t want to have to pay for another $3000 headlight or $5000 back seat A/C evaporator out of my own pocket.
While not my cup of tea, you have to hand it to them its probably the easiest way to do a drive train swap from a modern car and they drove it, which is better than most that never get off the ground.
Also, stuff for our cars is getting hard to find and more expensive when you do.