I had the same problem, seems the wg mount pad is too close to the exh. pipe.
I went to home depot, and got four bolts with regular hex heads on them, that way you dont have to use an allan tool, but rather a combo wrench, wich is thinner and will fit in there to tighten it up.
and on the one that wont go in, i used a little bit shorter bolt for that one.
I cant remember the sizes but they were metric, and came in individual bags at home depot, i just took my waste gate with me, and one of the allen bolts with me to make sure i got it rite. the 3 were the same length as the on3 the shorter was the next size down.
Current: 04 Gt, sonic blue, 3.73s, 2.5" custom o/r exhaust, diablo tune, cai.
Sold: 90lx, stock short block & cam, TFS 170's, 1.72rr, typhoon intake, on3 kit 70mm, AOD, 4.10 gear, cobra 39s, C&L maf, stock tune, 11psi boost