Scott,
I would chime in about installing the cargo floor, but whatever I say will be redundant. The only thing I would suggest is to protect the paint on the sides when you're sliding the panel in...and it's probably easier with the tailgate off, but if it's already on, you can do what Mich had suggested about leaving one of the cables off.
As for the '78 Pontiac taillight lenses...good luck finding them. I thought the Euro lenses were hard to find. I don't recall ever seeing those on a car. However, they are cool and unique...and you will have to reconfigure the taillight harness...or find a '78 harness.
The Euro lenses are also done in the '78 configuration.