First off, LS6 injectors might not be enough. I'd hold off on buying them until you get it tuned with the new cam, and see what kind of lb/hr you'll need. Also, by a 20th do you mean like .020" Decked? What CC combustion chamber would that yield?
Standard lifters are good fairly high. I have LS7 lifters in my camaro, and it Pulls HARD to the 6750 limiter.
Cam size generally does not denote the need for solid lifters. That is more of an RPM - based decision. ALSO when spinning to high rpm, or with a big, high lift cam, you really should go to a dual spring.
PTV clearance can vary greatly from cam to cam. It also has a LOT to do with the LSA of the cam, and the lift, not just the duration.
For what it's worth, I run a 62cc AFR 205 head on my Camaro (stock 2002 bottom end), with a Vengeance VRX4 cam (228/230 .588 /.591 on a 112 LSA) , and i had PTV issues. I had only .007" on the intake and ~.085" on the exhaust. I ended up flycutting my piston for the intake valve .093" to yield the recommended .100".
Here is a GREAT Sticky pulled from the GURUS over on LS1tech.com
http://www.ls1tech.com/forums/generation-iii-internal-engine/327734-cam-guide.html
Read up, and good luck!