Are you replacing your motor mounts? It is a good time to do it obviously (give them a coat of paint before they go in).
I used the Remflex gaskets once - once. They sealed well, but I did not like that they really fell apart when I had to pull the headers off once. Now I use the fel-pro copper gaskets and they work great. They cost about the same as remflex, but are fully reusable and seal perfect. They also install easy and you don't have to wory about crushing them.
I never installed my headers prior to dropping the block, just too much bulk to keep track of. I found it easy to install the headers from underneath the truck.
A couple items that you absolutly need to install prior to dropping in are the cruise controll bracket etc that bolt on to the rear of the heads.
Good luck,