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

I'm not really sure what you've been thinking about while planning this for 3-4 years but frankly you should have come to the conclusion pretty quickly that an 1,100 + lb engine in an A body is not a good idea. You can theorize/rationalize/explain this all you want but it's just not a reasonable endeavor. What is your vision for this thing besides just "doing something different?" If you're after tons of torque put a stroked 400 in it with stealth heads. It will weigh nearly the same as an iron head small block. If you're after the turbo, put a 360 in it with manifolds and go in the 9s. I'll put you in touch with a guy who can tell you how to do that.

A '73-up Duster weighs 3,400+lbs with a small block but it's going to be over 4,500 lbs when you're finished. Putting a Dana in it will add an extra 100 lbs to that. These cars were never meant to be that heavy, the chassis is not robust enough to handle that kind of mass. Again, a Duster is a "compact" passenger car, not a heavy duty vehicle with a rigid, traditional frame.

Not only that, as has been mentioned already, the front/rear weight bias of these cars is not so great to begin with. By doubling the engine weight over the nose and adding more support, it's going to be more like 75/25. Additionally, a 6BT is a really tall engine so the center of gravity is going to be much higher than it should be. You can probably forget about a hood too.

Just so you're aware, a 29" radiator is not fitting in an A body. Standard width is 22". With a 28" radiator half of it will be behind the radiator support panels with no air flow. Or are you cutting the radiator support out too?

That giant Mercedes truck trans is not fitting in that chassis without major alterations either so you're going to be cutting apart major structural sections of the car without any real sound engineering behind it. Forget that the chassis is also 50+ years old and probably fatigued from years of use and abuse. Understand that guys can barely fit a Tremec in these cars without major surgery so don't think you're just going to cut a little here or there and drop it in like it was meant to be.

Got a chassis jig to keep things straight when you start cutting and welding? If you're not a skilled fabricator or work in a body shop with pro-level equipment (does not sound like it), you're going to be in over your head. All that cutting and welding will put a bunch of heat into the metal, it's never going to be straight. It will also work harden when it flexes making it brittle and prone to cracking. You'd be better off having someone put a tube frame in it and work from there instead of modifying the original chassis beyond what's reasonable.

I'm not saying that you can't do this dude, I'm saying that you shouldn't. It's a misguided idea at best and will not be safe. Save the diesel for a truck where it belongs.