For my wife's 70 Coronet(happytoes24) we've used a combination of eBay, Craigslist and this forum.
I found a wrecked 2005 300 on CL for 600 dollars. We bought it just for the interior (front and back seats, center console dash, wiring harness, computers, etc). It was a wrecked car that had the top cut off because the fire dept couldnt get the doors open. The body shop that listed it removed the 5.7 and trans, left the transfer case for the AWD and other parts they didn't want. They sold what parts they could then put it on CL. This was way cheaper than getting bucket seats for a 70 B-Body. The 300 needed wheels since the body shop was going to sell the DUB wheels that came on the car. It's DUB Edition AWD 5.7 300C. So I turned to CL and found Dodge Charger 17" wheels with tires (tires are in no shape to be on a car that is driven) for 150 on CL. Just need them to push the car in and out of the garage.
She wanted a 6.1 and I issued her a challenge. I said she couldn't find a 6.1 for anything less than like 5k. Well she accepted the challenge and beat me by finding one on eBay (
http://www.chuckandeddies.com/) for 2500. Complete engine with wiring harness and cores for the alt and A/C compressor.
She doesn't drive cars with automatic transmissions. So thanks to lxmodguy and his ever changing project I bought a 6 speed off of him out of a challenger. Compete with clutch. I forget the exact price, but somewhere between 1500-1800.
Since the plan is put the Coronet over the 300 and since it's getting a 6.1. The stock rear end from the 300 (AWD 5.7) would probably break from the 6.1. I turned to CL again and found a complete IRS (brembo brakes, rotors, shocks, craddle, etc) a guy was selling from a SRT 300 for $1000.
We've got about 6k into this so far. Not including the Coronet (2500). So you can see there is no right or wrong way to go about this. I would recommend you first lay out what you overall goal is. This will help you determine the parts you'll need and the best way to go about it.
Best of luck