moving part of firewall back 3 inches and want to keep A/C?

[540sx]

I am putting in a 327 chevy and I got a 7 quart road race oil pan.I need to move the enging back about three inches in order to clear the power steering rack and the lines attatched to it. This in itself is not a problem, I know where I need to cut inorder to move the center area of the firewall back three inches.My concern is that I will be moving in to the area of the a/c and heating in my dash board.

Does any body have experience with moving the firewall backand keeping or not keeping a/c?I need to keep a/c but not necessarily the heating. I should have used an LS1.