Being it is a 318 Rallye, it is on the fence as to have a better resale with a 318 vs a 440. If it was a 360 Rallye, the answer would be clear, keep it OEM as possible. If it was a standard Challenger, a 440 would likely have a better resale.

Although there are buyers that would prefer a 440, I think most prefer OEM or at least an OEM look. If you make any changes, make ones that are easily reversed.

If it were mine, I'd pull and save the original drivetrain, pick up another 318, stroke it to 390 and detail under the hood to appear OEM as possible like a12rag's Satellite above.