If you plan on using an older trans you need to oblong the flywheel holes slightly to match up to the converter. You will also need to extend the alignment snout on the torque converter, you can buy that for abour 20 bucks. you need engine mounts I recommend going to ict billet they make things pretty easy for the swap and they give you lots of swap info. When it Comes to the mechanics side of things.
As far as wiring you can go with a holly set up ot modify your existing harness. It's a good amount of work to get everything working. I have done a few. Lots of information for wiring on lt1swap.com
You will need to update your fuel system. I recommend doing the suburban or blazer "fuel injected" because you would want the tank with the sump. tank swap and running an in tank pump. You can do an inline pump. I have done both but i don't like thr noise of an I line. That being said the inline pump is the easier route.