budget street motor 360 or 383 for 71 duster
If you go big block do a 440, in my opinion the conversion required to go big block is not warranted for 23 cubes. 440 on the other hand is a whole other story.......
I still would not rule out the 360, have you seen what a decent 360 will do?
Enthusiasts often get into the mindset of needing something bigger and better, I know Ive been there myself. Often what you were looking for is what you already have but don't realize it.