Sorry I'm late, looks like the big wheel haters are already trying to force their archaic nonsense on somebody else because they still haven't figured out how lousy their BFG T/A's really are.
What size tires are you looking to run? A '74 Duster can actually run a ton of tire with the upgrade to 17" rims. Up to 255/40/17 in the front, and as much as 275/40 or 275/45/17 out back if you want. 255's out back would be pretty easy, the 275's will be tight and might take a little work on the quarter lip depending on body tolerances.
Not sure what your exact plans are for the car, but it based on the set up you've shared so far, it sounds like you're setting it up for better handling. If that's the case, you have a lot of options if you go to a 17" rim. In the back the 8.25" axle is narrower than an A body 8 3/4, so be careful who you listen to for backspace. What works on a BBP or even SBP 8 3/4 will be different for an 8.25", even with the Jeep disks in the rear adding a little to the track width. Are you still running factory disks in the front?
In the front, you have a ton of options. Your limiting factors will be the fenders on the outside, and the outer tie rod on the inside because of the 17" rim diameter. That limits backspace to about 5.6" on most rim designs. Assuming you're using the factory BBP, 73+ disks you can use any of the following-
17x7 with anywhere from 4.25" to 5.6" of backspace, or +8 to +38mm of offset (so those Motegi's will fit on the front). Tire is limited to a 225/45/17 or a 225/50/17, the first will be kinda short (25"), the second a little tall (25.9")
17x7.5, 4.75" to 5.6" of backspace (+10 to +35 for offset), with a 235/45/17. Or, you can run a 245/45/17 on that rim also, but you'll want minimum 5" of backspace for that (+18 offset)
17x8, 5" to 5.6" backspace (+12 to +30) with a 245/45/17. You can also get a 255/45/17 on that rim, it will be tall at 26.1", but should work if you stick to 5.25" to 5.6" of backspace (+20 to +30 offset)
In the back the 8.25 is the same width at a BBP 7.25. I was able to get a set of 275/40/17's on mine with a 17x9" rim and about 4.75" of backspace, but it was super tight on the springs in the factory location. You'd have a little more room though, because those disks widen the track width some (about 5/16" per side). So...
17x8 with 4.25 to 4.5" of backspace (-6 to 0 offset), 245/45/17's would fit pretty easy. 255/45/17's would fit too.
17x9", 4.5 to 4.75" backspace (-12 to -6 offset), 255/45/17's. That rim is a better fit for the 255's, less sidewall squeeze. You may need a small wheel spacer on the inside, but with the disks maybe not.
17x9, 4.75" of backspace (-6 offset), 275/40/17's. This is a tight fit, so tailoring with a spacer or trimming/rolling the quarter lip may be needed depending on your ride height. They'll fit, but I wouldn't suggest it unless you're willing to do some trimming, especially since you're sitting fairly low with Hotchkis springs in the back.
Got me while I was still typing my response!
Everything for 17's is above, 18's are definitely easier up front because of the tie rod deal. I run 275/35/18's up front on 18x9's, 285's would take 18x10's and some more push on my front fender for sure. 275 up front is the limit with relatively stock bodywork :D
