I'm Cummins swapping a '74 Duster, thought I'd share the progress.

I agree with Kern Dog that this idea is ill advised. Just because something can techniclly be done does not mean it should be done. Putting a Cummins into a D150 or something would be a natural swap but shoehorning one into a smaller passenger car makes little to no sense. It's not a passenger car engine, it's a truck engine made to haul heavy loads. You're also going to have a rev limit of less than 4,000 rpm which does not sound fun.

But besides the chassis stuff, how are you going to cool it? Those things run pretty hot (215-220) and could potentially melt stuff under the hood without proper ventilation. You'll also have to science out your fuel system pretty thoroughly to run diesel. And what trans do you plan to use? Whatever it is won't fit without cutting apart all kinds of structural pieces of the chassis. So not only are you adding a bunch of weight up front but weakening whatever is behind it.

I'd think this through a little more before you start cutting stuff making the car un-sellable should you decide to abandon the project before its finished.
You'd be surprised what you can do with less than 4000 rpm, as far as cooling, I know I can certainly decrease cooling capacity most of the cooling is for heavy hauling anyways, unloaded I've never seen my personal truck nor my work truck exceed 198°F, granted, I'm not doing a lot of hot rodding with either of them but 100+ degree ambient temp in the summer they still run pretty cool. I'm hoping I can just put a 29 inch cold case radiator in it, but at the moment I'm struggling to find any way to calculate the heat load and cooling needed to keep the engine happy.

The fuel system won't be too terrible to figure out, if I use an air dog or a fast fuel system I'll be set. might have to get some bungs welded into the stock tank to make it work, and really all I need is just a level sender for the gauge cluster.

I've got a G56 set up to go in the car, it's a big girth transmission and it will require one hell of a transmission mount, but whatever structure will be pulled out of the car could be made up with the transmission cross member and additional supports around it