Let's identify your rearend. Sounds like it is the original 8-1/4". I'm in the same boat as you as I have a 1973 Duster with a 15x8 Rallye, 4.5" bs, 8.25" rear. I am using much less than 1" spacer. It's 3/8" iirc. Stock spring location. I have 255/60-15 (27") which is max for us.
My remedy is to upgrade to the wider 8.75" rear. There goes the spacer and room for 275/60-15 (28"). Then, maybe custom 15x9 with a 295/50 (26.65).
Offset Spring Hangers are another option for you to keep the 8-1/4 or use them in conjunction with a wide rearend.
View attachment 1716273395View attachment 1716273396