Donor 318 car for $500 or 383 4-Speed for $1000.00??

Does the four speed with the 383 include clutch pedal, linkage, etc.? That would cost you some too. Plus adding the hump in the floor for the shifter, a new drive shaft, heavier duty suspension parts, big block k-frame or shumaker motor mounts. More work for sure, but there would definitely be more muscle. Even with the work I'd be tempted to go big block if there is nothing wrong with it. The 318 sounds like it will need work sooner or later.