If you are going to have air conditioning there is a case that can be made to use an Art Morrison subframe with the stock LSx accessories, oil pan and off the shelf C5 brakes to offset the cost of the subframe. This has been discussed on pro-touring.com:
http://www.pro-touring.com/forum/showthread.php?t=45932
This is the approach I took on my 69 Firebird build.
Don