Oh, here are wheel rates in lbs/in for various A-body torsion bars. First number is bar diameter, second is control arm length (just ignore), and the last it the wheel rate. As you can see going from .850" bars to .870" is only a 10% increase and .890" is 20%. .920" bars are 37% stiffer than .850s. My 1.04s are 124% stiffer than .850s.
0.830 13.5 91
0.840 13.5 95
0.850 13.5 100
0.860 13.5 105
0.870 13.5 110
0.880 13.5 115
0.890 13.5 120
0.900 13.5 126
0.910 13.5 132
0.920 13.5 137
0.930 13.5 143
0.940 13.5 150
0.950 13.5 156
0.960 13.5 163
0.970 13.5 170
0.980 13.5 177
0.990 13.5 184
1.000 13.5 192
1.010 13.5 200
1.020 13.5 208
1.030 13.5 216
1.040 13.5 224
1.050 13.5 233
1.060 13.5 242
1.070 13.5 251
1.080 13.5 261
1.090 13.5 271
1.100 13.5 281
1.110 13.5 291
1.120 13.5 302
1.130 13.5 313
1.140 13.5 324