Tires and wheel fitment for a 1976 Dart Sport

I went to the Discount tire size calculator and plugged in your tire size and it calculates a width of 10.83" and mine at 9.25". Yes, they are 3.75" backspace. Will these work on the rear?

Yes, a 275 is wider than a 235, but I'm not running 3.75 of backspace on an 8" rim. I run a 18x9" rim with 6.1" of backspace. That's an extra 2.35" of backspace, and even by your calculator my tires are only 1.58" wider. The difference in offset between my rims and yours is a whopping 45mm, your 15x8's with 3.75" of backspace have an offset of -18, my 18x9's are at +27mm. With a 15" rim though you're limited to around 4.5" to a max of maybe 4.75" of backspace because of brake and ball joint clearance.

As far as the rear goes, yes, those rims should work if you have an A-body length rear axle in the car. You might even be able to run a 245/60/15 in the back still, but that would be as wide as you could go. To keep things in perspective though, a 275 is possible in the back with all of the stock equipment on a Dart Sport, so even with a 245 the backspace of your rims is limiting your tire options pretty significantly.