Looking for advice. Want to build a Duster to run on a budget.

The magnum swap would be the most cost effective, by far.

But $4k is going to be very light. Even if you get the engine in running condition, most "rollers" are going to need full suspension rebuilds to be roadworthy. And even with the cheapest parts and doing the labor yourself you'd have half your budget or more just into that.

Also, the later overdrive transmissions are not a straight bolt in. They need crossmember and tunnel work to fit, more/less depending on the exact model of transmission used.

I hate discouraging people from getting cars back on the road, but putting a Duster roller back on the road with a total budget of $4k is a big hill to climb. Maybe not impossible with the proper horse trading and free labor, but realistically I think you're going to have more into it than that if you're successful.