71 440 or 74 360?

You can get 400 HP from your 74 360 but it will take quite a bit of dollars compared to the 440 and installing a 440 in your Duster isn't really that big of a deal. As Jim said no cutting is required. A big block K member is all that is required and it will bolt right in. Also to echo earlier comments a 400 hp 440 will be much more streetable than a 360. In fact a 400 hp 440 is extremely mild and would probably get better fuel mileage if that is any concern. The tire shredding torque of a mild 400 HP 440 compared to a 360 would be fantastic.

And you would need a 727 for the 440. Your small block 904 won't bolt up.