Ready to get the Duster on the road. What is the biggest tire and wheel that will fit without mods? it has a 4.30 gear with a mini spool and a stout 340.
it does have an A body housing with resplined truck axles and BB pattern. I have 2 nice Police Car wheels that I would like to use.What do you suggest?

275/60x15 on a 15x8 with 4.5 or 4.25 BS (depending on how you measure that) is pretty much the default size.
I've done that on a couple Dusters.
There's some variance in panel fitment, though, from car to car and even side to side, one side usually has more room than the other.
Generally it's tightest at the front bottom corner of the quarter, I had to use a 1/4" spacer on one car between the ft spring hanger and where it mounts to the car.
If the car sits lower than stock, check the inside of the tubs where they narrow.

I have 275/60/15 M/T ET Streets on Dad's 72 Duster...

You'll need more wheel width than the cop car wheels have for something that size...

Mine are 9.5" wide but my backspace would not be correct for your car using the factory A Body 8 3/4"...I used a 65 B Body 8 3/4...

With the police wheels you can probably fit up to a 255/60/15.

The truck axles and whatever brake parts you use will determine your back spacing requirements since the drum face will likely be farther out towards the wheel lip than the OE small bolt pattern/2" drum shoe measurement. Mock it up with the drum in place, put a wheel on and hang a plumb bob at a few points to see what kind of room you have.

Fitting a larger wheel/tire combo can be tough without doing any modifications since the limiting factor is usually the location of the leaf springs. Move the springs in and you can fit a 275/60 15 tire with ease. Some guys lift the rear with air springs to gain more clearance but then you have a goofy stance and tires that stick out.