need help purchasing street radials for my 74 dart sport

Everything "should" work
I'm a little nervous of the 4" front wheel backspace. I know 3.5 will work . But by 4", alloy wheels are getting daymn close to the UBJs . And, the rubber may hit stuff at full lock, depending on where it is in the suspension cycle.
If you have front disc brakes,make sure those front wheels are designed for them.
Hang around for a while, other more knowledgeable fellows may show up.
I have a 68 Barracuda, which most guys say have the same inner tubs as Dusters and Demons.I have been able to fit 325-50/15s into those, for straight-line action; but I have to behave myself around turns. 295/50-15s are anything goes, if properly fitted.
275/60-15s with up to a 4.5 bs, even, are easy . These are 28" tall and fit into my Barracuda tubs. I have heard that they don't always fit. The easy fix is to just move the axle back a bit.