Installing bed sills...on or off truck??

[airbrushguy]

I removed the bed from my '71 stepside while i was restoring all the rusted out other parts of the truck. While the bed was standing on its cab-side end, I removed all the bedsills and the rotted wood.
Now I'm getting close to putting the bed back on the frame, but should I bolt the bedsills onto the bed before I drop it down or after?
I'm an ornery old fart that doesn't like to ask others to help me, however I will have to get some young bloods to help me with lifting and placing down on frame. I don't want to ask them more than once if I can help it.
If it's already down, the outside edges will need spacers or something to help me install the wood slats between the sill and the edge angles.
Anybody have experienced help as to how you did this?
Thanks