How Tall a Front Tire Can I Go On My 74 Duster?

IMHO, installing anything fatter than a 215 on a true 7"wheel, would be a waste as at proper tire pressure, it will push the center ribs up, and they will take all the wear. Therefore, to get a long tire life, you will have to begin to reduce your tire pressure. and away goes your handling.

and BTW
Have you run those 4"bs wheels before?
I tried that on my 68 Barracuda, and 235s come dangerously close to rubbing either on the Strut rods or on the Frame , just at ride height, never mind at full jounce/rebound or most importantly, while braking.
I realize Your Duster has a slightly wider track, so maybe it won't be an issue; still, you might want to consider it.
Whereas to "how tall"?
215/70-15s math to about 26.9 tall, which is pushing it.
Personally, I'm not a fan of that look, but it does make charging over speed bumps easier on "under the steering" headers.
and Lord knows I love to charge those things, lol.