Gs were 8.25 treadwidth
To get a similar section width of a G, in a P-metric the sidewall bulge depends on the rim width, but a close guess might be 245 on a 7" rim; but the tread will not be 8.25, because radial tires are not made the same way anymore.
To get an actual 8.25 treadwidth on the road, you might need a 275 and your choices are 275/60-15 and 275/50-15. Both of these need a 9" rim width to run flat to the road. The 50 series is more critical. I have run them both on an 8.5rim, but the tire pressure was quite low to flatten them out.
IDK why you are so concerned by the height of the tires but here they are;
275/60-15 is about 28inches with a sidewall height of 6.5 and the 275/50-15 is about 25.8 with a sidewall height of ~5.4 These are the minimum rear tire sizes I would consider for a BB Duster. The 275s will easily fit in a Duster tub, but not on any off-the-shelf wheel, I don't think.
There is probably more going on here than you might know. I don't know how much driving experience you have, or how much of it is behind a short wheel-base 440 equipped car, but;
With the skinny tires you are considering, even a 318 will get into trouble when you push it in a turn....more so if you have a limited slip. With both tires hanging on the edge of spinning, in a turn, as soon as they do spin or slide, you only have microseconds to stop the spinning before the back end comes around. I kid you not.
But with a 440, you have probably twice the torque the lil teener makes,and a huge more amount of inertia in all the spinning engine parts,plus the pig-heavy 727 convertor, plus all the spinning parts in the 727, plus the rear wheels if they are spinning; so you have as good as NO chance on skinny tires, to recover. When the back end lets go, you are instantly out of control. And you are on a circular path to doing a 180* unless you hit something first.
And you cannot even brake your way out of it, because
#1, that is 100% the wrong thing for the back tires to be doing, and
#2 with skinny front tires too, you haven't got any stopping power anyways, and #3, if you hit the brakes too hard during a wipe-out, so what? your front end is not going in the direction the wheels are pointing, and if you lock them up, now you are like a curling rock on ice, and whatever direction you are sliding, your car will continue to slide until those 4 tiny rubber patches finally bring you to a halt ... or you hit something.
#4 the first time it happens, I don't care how fast you think your reflexes are, yur going around. Your mind will be going a million miles per hour, but you'll be making all the wrong decisions.So you better put some practice in on an empty WET parking lot.
I'm not making thischit up, I have 50 years of experience of wiping out, lol, beginning in 1970, grade 10, with a 1970 Swinger 340/4speed/3.55s, on those pathetic, factory-installed, E70-14 Polyglass roller skates. At 30 mph,I used to change lanes by whipping the steering wheel over a lil and then whipping it back; the car just slid right into place, exactly one lane over; man those tires were junk!
I beg you not to install these skinny tires on your 440 Duster.
One time ,in traffic, I switched lanes a lil too fast, wiped out, and ended up on a driveway at about 90* to traffic. Man I couldn't get outta there fast enough, cuz what I thought was a driveway was actually the gateway to the municipal cop shop, and those guys all knew me pretty well, and they all didn't much like this punk-azz schoolboy. BTW; my Swinger was PantherPink, so there was no hiding it,lol.
