First, welcome from Alaska.
Second, nice ride. Looks like it's already been lifted.
Tell us more about the truck's options (or features, powertrain, etc.) so we can better help advise you on your goals.
I haven't seen a sbc 400 in years. It may be easier to build a 383 stroker from a 350 than source a 400 and build it. Ther are many sources for stroker kits if you decide to go that way.
For an overdrive trans you'll likely want a 700R4 trans although IIRC, the later models are better.
Your bed doesn't look too bad in the photo so you may not need to replace it. Various bed parts can be purchased through the store on this site. Here's a link:
http://store.73-87chevytrucks.com/merchant2/merchant.mvc?Hopefully, this will give you something to start thinking about. Others will be along soon.
Rusty
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