Mike spent a bunch of time and $$$ on his LT1 and never got to 400rwhp. With a cam and a few minor mods, he has 400rwhp with the LS1....food for though. Plus, maybe the biggest factor od all....no Optispark!!
Coilpack ignition is a much better option (LS1).
I have the LT1 T56 in my Z, and it's a great engine, but for higher HP requirements, all I'm saying is that learn from others experience, and then decide. If I were to do it over, I would go LS1.