245/60/15's on 15X8 rims

Correct... if you had WEAK SAGGY springs there will be an issue .. and why are we sticking with 4.5 backspace ... you can get different back spacing .. like i said with CORRECT back space ... 255-60-15 will fit

Man I'm tired of this. No, with the "correct" backspace you still can't fit 255's on every Dart. That's just BS. Yes, with the correct backspace you can fit 255's on some Dart's. Maybe even most Dart's. But not all Darts. Capsmash all you want, it's just not true. I posted examples of guys that had rubbing, that had to cut and roll the quarter lip on one or more sides of the car. The examples I posted also showed they had the right backspace. Just do a search, there's posts all over this board of guys that can't fit more than a 245 on their Dart even with the correct backspacing. There just isn't enough room on every Dart that ever left the factory. Some have shifted axles, some have body tolerance discrepancies, it's just how these cars were built.

Why am I recommending the OP use a 4.5" backspace? Well, do the math. 4.5" backspace, 1.1" of tire overhang, that's 5.6". He has 6" to the springs. That only gives him .4" of clearance. Now let's do the front space. 3.5" of front space, 1.1" of tire, 4.6". Why, that's .4" of clearance. It's almost like I meant to center the wheel in the wheel well! And realistically, the section width will be squeezed down slightly on a 15x7", so he'll have the 1.2" of clearance he needs on both sides.

Can you run the clearance to the springs tighter than that? Sure. You could go with 4.625" of backspace if you wanted, you might or might not need a small spacer. Probably not. If you went 4.75" you'd need a spacer, but if you were thinking of going disk brakes later and don't want to buy new rims that might not be a bad idea, run the spacers until you do the disk conversion, most of those add about a 1/4" to the track per side.

But the "correct" backspace for the measurements that 68 Coronet RT is giving me? I'd say it's 4.5". I might go 4.625" for custom rims, but I like to err on the side of too much backspace and correct with a spacer if I have to. But some folks don't like messing with spacers.

I have 4.25 backspace. A 1/4" isn't much but it might be rubbing or not. I have a little room on the inside. The 4.25 b/s was a 'off the shelf' wheel.View attachment 1715155764

Right, but you have SBP axles and brakes. The BBP conversion widens the track width about 5/16" per side (smidge more), from 57 1/8" drum to drum to about 57 13/16". So actually, the extra 1/4" of backspace on the wheels for the BBP axles and brakes we're talking about should give a touch more clearance than you have.