273 for 318 swap advice

Sounds like the extra six or seven hundred pounds of the Duster over it's mid 60's predecessor is what's doing you in, so the 45 extra cubic inches of a 318 sounds like a good answer if you're not willing to run a loose converter or change to stiffer gears.

Rods, not the crank are probably the weak spot, and if the 318 is a '67 it should already have a steel crank. 308 casting heads from a 90-91 TBI pick up are considered the best bang for the buck, provided you don't have to dump a lot of coin freshening them up, and you make sure your pistons are zero deck or close to it.