Even tho it would be nice to have a crate 460 I wouldn't go that way on my first build. Turnkey only means it's got a carb, distributor etc. Ready to fire up. Cam already broke in etc.
THen you need a tranny, torque convertor, starter, motor and tranny mounts, headers and exhaust, a shifter, a better rad, maybe better brakes, rebuild the suspension, better rearend, driveshaft etc etc. Add another 5 grand.
It adds up in a hurry and becomes a massive budget beater.
Keep doing research, take lots of time, go to car shows and cruise nights and look at similiar set-ups in trucks like yours. You'll be farther ahead.
I'd look for an old LTD with a 460 auto and build from there.
If you buy the big motor for 5 grand then odds are a year or two from now you'll have a big motor sitting in the corner and no truck built. That happens all the time.
Save your money. Do it right!!