Will a 727 automatic transmission with slant 6 engine in 1971 duster hook up 340 engine?

No, a 727 automatic transmission behind a slant six in a 1971 Duster will not directly bolt up to a 340 engine without modifications. While the 727 is a relatively common transmission used in many Mopar vehicles, including those with the 340, the flexplate (or flywheel if it's a manual transmission, though you specified automatic) is different. The 340 has a different bolt pattern on the crankshaft, requiring a flexplate specific to the 340 engine. Simply swapping engines would leave you with a mismatch, preventing the transmission from connecting. You'll need a 340-specific flexplate.