I see orange '71 'Cuda & seats - spoilers & louvers - you mentioned that it's a 340 4-spd car. Likely the top of the 340 food-chain short of a convertible - can you share fender tag info?
An iron headed 360/408 can be dressed to appear as stock as an original 340. Its impact on your wallet will be minimal, and its negative impact value/desirability of the 'Cuda would be minimal.