LS powered Duster, Abomination or Go For it!

Like everyone says; your car, your choice.
That "free" LS won't be so cheap once you figure in custom mounts, custom center sump oil pan and pickup, either a new 4 bbl. manifold or the supporting electronics for the factory EFI, custom built headers, transmission mount/ linkage/driveshaft fabbing, trans controllers, etc. etc. etc.
Personally, sell the LS and get a LA/Magnum and have fun. You said you don't want to fix it all up, just want to drive it like you stole it- can't do that if it's sitting in your garage waiting for custom parts or while you try to fab everything up.
Guys pop up here and elsewhere fairly regularly with this same proposal- 90% that actually attempt it shortly end up on Cracklist or Marketfarce; "just need finishing touches", "all the hard work done", "change of plans", "easy finish"... In other words, way harder and more expensive than they thought.
Your car, your time, and your money.