What would be the best engine to droop into a 1968 mustang?

What would be the best engine to droop into a 1968 mustang? - 1
Added (1). What would be the best engine to droop into a 1968 mustang? I'm planning on buying a straight 6 1968 mustang and I want to drop a V8 into it. What would be the best lowest cost engine that would drop into the mustang with no adapting? (I don't care is the engine is even a ford engine I just want more power)

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