340 or 440

Great question!

I have had both LA & B engines in a Duster. This is how I see it. Speaking strictly from a car that doesn’t see the strip but may, LMAO, may see a light to light or long stretch run.
But street driven is the primary usage of the car.

The big block provides nearly every advantage except;

Added weight. This is an apples to apples comparison. If you lighten up the big block with aluminum heads and intake, so can the small block. The big block is just heavier and at the worst place. Up front over the nose. Consider this when it is a driver and a little bit more so because it is all iron. Why? The extra weight effects handling and braking a little bit.

Is it dangerous?
Only to the stupid.
(Darwins Law may take affect in this scenario.)

Fuel mileage;

The bigger the engine the more fuel it consumes.

Fit in engine bay and it’s issues. This list can get long rather quickly. A quick list below that may not contain every last caveat....

B engines offer a little more clearance over the RB and this help header fit. Header design may be a hindrance for plug changes.

Upgrading the cooling system is a highly recommended move. Who runs stock big blocks in a A body? Power creates heat that needs to be removed.

Suspension upgrades increase road handling abilities. A certain plus for any car with any engine, the added weight of the bigger engine & trans just need a little more to bring it back to how it was before the swap.

Etc...

I guess your question to me having both engines in a Duster would really be, “Would you do it again?”

Yes IF it was a drag car with limited street time.

This comes down to what I’m doing and of course how you the reader would want to go about doing it.

It is IMO that an A body with an LA (or new HEMI engine) is the best combo for what ever the goal is. Unless it is extreme drag racing.

When I had the B 400 in the Duster, it wasn’t a bear to drive or stop. The feel of the car and the braking distance of the car with its 4 wheel drums changed a bit so extra distance and a more cautious driving demeanor is increased.

All in all it was t a bad deal.
But I probably won’t do it again for a street driving ride. For something that will see the strip once in a while, monthly at a min. I say heck yea!

The small block is just easier, more roomy & can certainly provide excellent power and speeds as well as easier to get the car to handle better on the road.

As I myself gather parts for the Duster now, it will be small block powered. The B 400 will be moved into a B body later.

But this is just me. For you, don’t settle. This turns into regret later.