Just to check, took a WB mask "mask+orig" (25379 vox), and then dilated by 5 voxels, so there was a much smaller WB-like mask (i.e., similar shape), called "mask_shrunk+orig" (4130 vox-- prob I shrank it too much, but for this comparison it probably shows what is relevant). I then ran 4 test cases of 3dClustSim using your two sets of ACF parameters:

A) mask+orig with ACF = 0.513804 5.16813 12.352
B) mask+orig with ACF =  0.520152 5.1862 12.4322
C) mask_shrunk+orig with ACF = 0.513804 5.16813 12.352
D) mask_shrunk+orig with ACF =  0.520152 5.1862 12.4322

In each case, the command I ran was like the following (substituting mask names and ACF param names):

3dClustSim -nodec -both -mask mask+orig. -acf 0.513804 5.16813 12.352 -prefix mask_0.513804_5.16813_12.352

For space, I am just copy-pasting results from some of the middle-range, most relevant cases of pthr:

A

0.010000      43     44     45     47     49     52     55     60     65     76
 0.007000      33     35     36     37     39     41     44     47     52     60
 0.005000      27     28     29     30     32     33     35     38     43     50
 0.003000      20     21     22     23     24     25     27     29     32     36
 0.002000      16     17     17     18     19     20     21     23     25     30
 0.001500      14     14     15     16     16     17     18     20     22     26
 0.001000      11     12     12     13     13     14     15     16     18     21

B) same mask as A, but other ACF pars

0.010000      43     44     46     47     49     52     54     58     63     76
 0.007000      33     34     36     37     39     41     43     46     51     59
 0.005000      27     28     29     30     31     33     35     38     41     49
 0.003000      20     21     21     22     23     24     26     28     32     38
 0.002000      16     17     17     18     19     20     21     23     25     30
 0.001500      14     14     15     16     16     17     18     20     21     26
 0.001000      11     12     12     13     13     14     15     16     18     21

C) shrunken mask, same ACF pars as A

0.010000      22     23     24     25     26     28     30     33     37     47
 0.007000      17     18     19     20     21     22     24     26     30     38
 0.005000      14     15     15     16     17     18     20     21     24     30
 0.003000      10     11     11     12     12     13     15     16     18     22
 0.002000       8      9      9     10     10     11     12     13     15     18
 0.001500       7      7      8      8      9      9     10     11     13     15
 0.001000       6      6      6      7      7      8      8      9     10     13

C) shrunken mask, different ACF pars from A

0.010000      22     22     24     25     26     28     30     33     36     43
 0.007000      17     18     19     20     21     22     24     26     29     35
 0.005000      14     15     15     16     17     18     20     21     24     28
 0.003000      10     11     11     12     12     14     15     16     18     21
 0.002000       8      9      9      9     10     11     12     13     14     17
 0.001500       7      7      8      8      8      9     10     11     12     15
 0.001000       6      6      6      7      7      7      8      9     10     12

Summary:
+ for the same mask but different ACF pars, the results are quite similar, esp. as smaller p
+ for same pars but smaller mask, the cluster sizes are uniformly *much* smaller (as expected).

--pt