[jocko]

Like truckster said, your truck should've come with a TH400. Pic below will help you confirm based on trans pan shape.
Also, if your carb looks like pic 2, it's a Q-jet - but note that it will also have quadrajet embossed on it somewhere. If you post the carb's part # (you can see this one's the driver's side just inboard of the throttle linkage on the carb body), we can tell you if it's likely original to the motor.
I'd keep that 402 and rebuild it or get it rebuilt (but yes, to answer your question, pretty much a direct swap for any Gen IV big block). Your truck is worth more with it if you ever decide to sell it, and, considering the fact that most had a small block, it's more than capable of any task you're planning to undertake with it.
Also, for what it's worth -a 3.54 Dana 60 likely is under your truck - unless the SPID calls out a 4.10 or other optional gear. You have a great combo. If highway rpms are too high for your liking (which may not be the case if it indeed has the 3.54s), I'd still keep the truck as original as possible and add a Gear Vendors over/under drive. Then you'd be set for anything, to include cross country cruising! Show us a SPID pic when you get a chance!