Roller Cam Specs for 340

Ok well then, you have a legitimate reason for 60-over, I get that. and, you have given good thought to the combo.
The automatic is a wise choice.
255s will be your limiting factor, unless you address those traction issues. Your Duster will accept up to 295/50-15s on 10s, inside the tubs, even lowered, but getting them installed on the factory rear end is gonna require the exact right backspace, which will likely require custom wheels. Your other option is to buy the wheels you like, with a backspace up to 4.5inches, move the springs over, and have the rear-end narrowed to fit them, which is the solution that I chose. and even at 275hp, yur still gonna need a traction aid.
As for cams, I had a 223/230/110 high-lift Hughes/Engle cam in my 367 for several years, which, I was quite fond of. I was quite sad when it dropped lobes. However,
I ran that with alloy heads at up to 195psi, lol.
By the cars trap-speed, that combo was making ~335hp.
After that, I installed a 230/237/110 hi-lift cam, which, by it's trapspeed the Wallace says is now making 430hp. This cam lost a very large amount of bottom-end torque and at first I was very disappointed. I solved that with a lower first gear in the Trans. But the lousy fuel-economy, even with overdrive, that I could not do much with.
If I had a 340, I would likely stick with something like a 228/234/107. The split would allow you to move the cam around some without destroying the overlap cycle. The generous overlap would let you tap into a good amount of top end power. the comparably long power-stroke will get you some reasonable fuel economy. To get some generous bottom-end torque, to run those 3.23s, or even 2.94s I would install alloy heads and get the pressure up to in the window of 185>195psi, which, in my case still runs on 87E10, at full Power. timing. That's what I would do.
Except, I would run a 4-speed, cuz that's how I roll, lol.