You sure that terminal is rated for that much current??? the contact patch is pretty small. Not sure I have ever seen a faston on an 8 ga wire and thought that 1/4" Fastons max out at 20 amps. Usually by that size, a different connector style is used, such as a ring terminal. I would ABSOLUTELY NOT run 30-40 amps through a 1/4" faston.
The fact that you cannot find a die in that size should be a large warning sign, as I don't think that style of crimp can carry that kind of amperage either. terminals for this size of wire are usually full sleeve.