I just did mine the other day. I fully installed the inners to the outers on the ground and then put them on the truck as one piece. It worked perfectly! And because I used repro inners, it was a big help doing this off the truck to get all the bolt holes lined up.
I painted the inners body color so I didn't want to scratch them all up doing it the other way.
------------------
63 Impala*66 Chevelle SS*69 Chevelle*69 C10 Stepside*71 Cheyenne 20*72 Chevelle
Parts for sale:
http://www.angelfire.com/trek/mr409/items4sale.html