Swap A833 for Auto - 71 Duster 340

A518 is the 727 version with standard 727 ratios of 2.45-1.45-1.00-.69od

The A500 is A999 based with ratios of 2.74-1.54-1.00-.69od.. the 1-2 split is a lil wider but if you gear it right, the split won't be a big deal.

Neither will fit into an A-body without widening the tunnel, and re-engineering the rear mount, and of course, you will need a shorter driveshaft.
Either unit is a great choice.
I'm liking the A500 with 3.91s. That will give your 340 a kickazz 10.71 starter gear, yet cruise with a 2.70 final drive, and 65=2180(with 27s). The 340 with the factory cam won't make great fuel mileage at 2180, but Ima thinking better than at 3160 which is what the rpm would be without the overdrive,lol.
I cruise my 367 with a 2.77 final drive at 65= 2240, and with a previous cam, the Hughes HE2430AL (270/276/110) cam, that thing did better than the best stock 318 I ever had.
The A500/3.91s will put you at about 32= 2400 cruising in second/5000 in first at WOT, and she'll hit 5500 at 34.4/5800 at 36.3 by the math. So that's not a lot of headroom for a footstomp. On the 1-2 shift, the Rs will fall to .54 or from 5800 to 2970, so a low-compression 340 with the factory cam is gonna struggle for a bit, but yur only doing 36 mph, so by the time you get to 40, the Rs might be up to 3600, and she'll be coming back up on the cam in a few milliseconds. Then she'll finish at 60=5000 more or less on the power-peak. She'll be about the best she can be, from zero to 60 with that combo.

TheA518 might be a hair quicker, but it will take 4.10s to equal the second gear of the A500. Your first gear with 4.10s will now top out at 5800=38.7, and on the 1-2 shift, the Rs fall to 59%, so to 3430 so that's up about 460rpm, I like it for the 340. And your cruise gear is 2.83, so 65=2280, heck yeah, the 340 cam will like that better.
So for a low-compression 340, I recommend the 2.45-1.45-1.00-.69 ratios with a 4.10rear, and 27" tires. And that means the A518 is your choice; or, you could put the A904 gears into the A500 case. I have not tried that, and don't know what parts would fit on the mainshaft, so you'll need to talk to someone who has.
Be advised tho, that in either case; low gears over 10/1 with a TC are killer off the line, so don't be leaning forward when you gas it, and hang the heck on to the steering wheel; cuz if it don't spin, you will be launching into tomorrow.. I highly recommend not to bolt on a Double Pumper carb,lol
With 4.10s, you would be able to swap out the 340 cam, for one a size or two smaller, which would pump up your cylinder pressure, and get back some low-end torque, which would increase your hi-way mileage, AND, you could swap out the 4.10s for a more sane 3.91. Of course a cam two sizes smaller would be able to pull the A500 wide ratios gears......
So, like said
the compression test will tell a lot.
Until then I'm just blowing smoke.