A Body Tire Clearance

topside said
'74 Valiant - so it's a Scamp, right ? Which means it has Dart wheel wells & openings.
They don't accept much with an 8x15 wheel, if that's the size you're planning.
Springs and wheelhouse and wheel opening restrict wheel/tire size to 15x7 & 235/60s on the Darts I had & a Buddy's '70 Dart.
15x8s I've used were 4.5" BS, the 15x7s were 4.25" BS (factory-sized BS).
Moving springs the small amount that the relocation brackets allow will help a bit, but not a lot - even on a Duster.
Ft of wheel opening restricts tire OD, but there's about another inch available if you cut them.
The right side on all of my '68-'74 A-bodies has been tighter than the left by about 1/4".
If it's actually a Duster, they have more room, and a 275/60 on 15x8 with 4.5 BS usually fits, but are pretty tight @ stock ride height.

There are several members running 255's on 15x7's with 70+ Darts and the springs in the stock locations. My own '71 Dart has enough room to do that. The backspace on the wheels has to be perfect, but it's possible on some cars. 245's should fit pretty much all Darts if the wheels have the proper backspace.

The 1/2" offset is usually good to go up to the next wider tire available on both Dart and Duster bodies because of how the tire sizing goes. On a Dart the 1/2" offset can get you to a 275 tire, again, there are several '70+ Darts on this board doing exactly that. On a Duster the 1/2" offset can get you to a 295 in there. Depending on ride height some quarter lip trimming will probably be necessary, especially on the Dart body. But I was running 295's on my Duster after the 1/2" spring offset and a bit of quarter lip trimming, and my car sits lower than factory. With super stock springs I wouldn't have needed to cut anything.

Your backspaces are all based on an A-body 8 3/4 with BBP axles, any other axle width would require different backspacing, including the factory BBP 7.25 and 8.25" rear ends which are substantially narrower than the A-body 8 3/4.