I did some research and actually bought cross sills. They are leaning against the wall in my storage place. I had to move them out of the master bedroom last year.
Here's a start on a parts list. The cost adds up quick.
Cross sills (I think I have 6 of various sizes)
Wood
Bed strips/sliders
Bulkhead or new angle to modify the existing bulkhead.
I just noticed you're discussing 73 and up. Mine's a 72 but no doubt the conversion is similar.