Check out my car and talk to me about 318/340 swap

I'm not a B body guy, but I'm pretty sure the 340 wasn't available in the '69 Charger... A body only at that time.
Swapping it in place of the 318 is easy as falling off a log. Use the 318 motor mount brackets- passenger side bolts right on, driver's side needs a ~1/2" spacer (stack of washers works just fine) between the bracket and rear mounting ear of the engine. Or dig up a 340 driver's mount. Drop it in.
If it truly is a '69 340, you'll need to swap the timing cover and cast iron water pump to the '70 and later timing cover and water pump for your Duster (the inlets switched sides), and you'll need to either remark your balancer or get a post'70 neutral balancer (timing marks also switched sides). Alternatively, get a pre '70 radiator.
Flexplate and convertor will switch right over from the 318 for the 904.