hi, i own a 2000 grand prix gt. i was wondering if i buy a new pcm off zzp, will it make a noticable difference with a shift kit.

yea, it will adjust yoru shift points, raises rpm, and increase the line pressure when you shift at WOT... And if you plan on adding mods(or have a few other ones) its a great addition to them, its seems that mods like the CAI and downpipe are pointless until you get a readjusted PCM...

is it hard to get to the back exhaust manifold to put headers on

You will have to remove the front mounts (dog bones) and roll the engine forward to get to the back. The engine should rotate itself back to normal but if not you will need to rent an engine hoist to get it back into position. Once you install the header on the rear tighten it. Leave the front header loose until you install the crossover pipe then tighten the front header and crossover. This will make it much easier to install.