Something in there rattling around in the rear...broken teeth or something in the muffler (baffles broken off). I can't tell which is rattling by your video
Check with your Harley Davidson dealership. Those guys get a lot of chrome done and it has to be top quality and everything they have had done for me was very reasonable.
I think I have missed something too. I have never done it that way and it is one of the things you never know until you try it. But it does seem like a lot of extra work.
Isn't the rear is too wide for your valiant? You can pick up a motor and transmission anywhere. You will have more money in getting the parts adapted to your car.