need help purchasing street radials for my 74 dart sport

I see no problem in the front.

But, it sounds like you are mostly concerned with wheel lip clearance in back. You are forgetting, or don't know that the tires have to clear the leaf springs. Pretty sure 255-60 would be ok but a 275 may get into the leafs. Cheap fix is wheel spacers. A 255 also matches your 8" rim width better. A 275 should have a wider rim.

You don't say what you are going after. Big tire drag look? Max traction?
My personal OPINION is a 255 looks best and a 275 looks forced.