You've said this "1,500 lb drivetrains in 3,400lb. car" thing twice now, so I'm assuming you don't understand that your math is wrong. A '74 Duster weighs ~3,400 lbs
with its drivetrain. He's not adding 1,500 lbs to the car.
That 3,400 lbs is with a fully dressed small block, which means you need to be subtracting at least 600 lbs from that figure, because we're talking about a fully dressed, all iron, factory 360 right? Maybe more if it's an AC car? And then you need to subtract about 120 lbs for the transmission. So what, 720 lbs worth of drivetrain?
And as I said earlier, the dry weight on a '67 426 street hemi was ~840 lbs. So, they put those in A-bodies right? With A833's? So, now we're subtracting 960 lbs if you're comparing an A-body with an early street hemi to a 5.9 Cummins?
The op is probably gonna be adding 700 or 800 lbs to the car vs say an early 426 street hemi or all iron big block, which no one would give him any grief at all over. Yes, that's a lot, but it's not outside of the realm of strengthening the chassis.
And we haven't even addressed payload, because I'm sure somewhere there's a spec for what the maximum payload is with 4 passengers and a trunk full of junk. So even adding 800 lbs probably isn't going more than a few hundred lbs over what Ma Mopar figured that Duster could haul fully loaded with passengers. I dunno about you guys, but I've never had 4 people in my Duster at the same time, so that wouldn't even be a thing.
And it's a '74, so there's a good 100 lbs worth of shock bumpers he can get rid of.
And right,
it's a '74, which no one gives a crap about anyway. At least that's what everyone says when you want to restore one right? That '74 won't be worth anything and you should get a '70-72? Definitely heard that before here.
For
@Daniel Garcia, the big question I have is why the G56 transmission? Since your car is still going to be substantially lighter than a 2500 truck, and won't be towing or hauling, I bet you could use a smaller transmission than that which would definitely make your tunnel and crossmember fabrication work easier. Unless you're planning on towing 10k lbs with your Duster too...