Forgive the off topic suggestion, but have you considered water/meth injection? A Snow kit on your car would significantly drop IATs and make detonation pretty much a non-issue. I'm sure you know all of this already, though.

Well, your boost numbers will likely go up with E85, but down through the intercooler, depending on how restrictive it is. At least you know the Dart will take anything you throw at it and beg for more.The 76GTS was making over 20psi on the dyno last time when the wastegate spring decided to not play nice.
If I remember correctly it hit 23psi, MOAR power!
I wouldnt worry about lag at all. My little Precision Turbo 76GTS .96A/R is the little turbo that could. I have been very impressed with it.
T4 base, small 3" DP and still makes silly boost on my 363.
If I had to do all over T6 would be the way to go.