you wanna get the rear engine mount WITH the STS. they compliment each other very well!
as far as the tune goes, just read this entire thread, it's only 2 pages long, it'll answer your questions and it'll save me from retyping the same information. the contact information for wayne and tom are at the end.
let us know if you have any other questions about this. also remember, all i did was a search in the SVT performance section for SCT and DIABLO and came up with your answer. it's best to try and do a search first.