Shermanthetank

So after thinking this over for the last few weeks, i am still at a stand still partially from lack of knowledge. I have a 2004 Silverado 5.3 with 90xxx, and for about 3 years now i have had a Garrett 60ar intake 80ar exhaust turbo and i am considering finally putting it on. I am planning on running a log style manifold, that i will more than likely fab myself since i can't find one i really like so far online. However i am still unsure of a couple things; one it is internally wastegated, and from what iv seen the lowest numbers you can get from internally waste-gated is about 8psi, i am looking for about half that. So how would i convert it externally waste-gated? Also i am considering running the turbo off only one exhaust bank, why, because fabbing two exhaust manifolds sounds like 2x the work, and i don't want it to spool at 2000rpms, thoughts?