I know the photos are with the car apart, but you get the idea. The booster will fit, I've seen it done before. I thought I'd post up some photos of the process and how I did it all.
I first took the old master cylinder off and removed all the pedals. I imagine you could do it without removing the pedals, but it would make it a lot easier and it isn't hard to do, plus you can replace the bushings and crap while you are at it. I replaced everything in the pedals and powdercoated them and installed Modo pedal covers.
The first thing I did was take the LS1 booster and made a template for the opening needed.
After this I set the booster in the firewall and set the template, then traced out the opening and cut the hole needed.
And then test fit the booster:
After this I took the brake pedal bracket and removed the studs for the old master cylinder. I decided to use the studs on the LS1 booster instead. I have heard they fit perfectly, but on the car I am building it was a little tight, so I had to grind the holes a little to make it work. Be sure you test fit everything before you get too far into it! You can see them removed in this photo of the bracket.
What I did next was weld in some metal to fill in the top part of the hole for the old master cylinder. Then I reused the template and cut the new metal to the correct hole.
After this, I reinstalled the pedals and installed the LS1 booster on the firewall. The LS1 booster has an eye hole connection to the pedals, where the 1st gen has a u-shaped bracket that screws into a solid rod. I installed the rod and bracket on the pedals and then measured the length needed to put the two together.