I suggest rethinking the size of the turbos as well. I spoke to several turbo companies before I settled on twin Precision 67/68's for my 418" LS motor. My motor is 131 cubic inches bigger than yours and these turbos on my motor should make 1200rwhp all day long and carry RPM to roughly 7,000.
Here is a 427" LS with Precisions 67/66 making 1143rwhp on 19lbs boost.
http://ls1tech.com/forums/dynamomete...1040-rwtq.html
The point I am trying to make is selecting a turbo based on CFM required for your power goals and selecting the smallest one to achieve that will most likely make you much happier when it comes to spool time in a truck.
Spool is sort of like converters or cams, each person has their own interpretation of how much "lag" is acceptable.