I just used one of those. The firewall gasket is thicker in the kit than the one sold by itself. Not sure why but it seals really well. All the other pieces are very well made.
How far do you want to get into this project ?
You can put the new gasket on from the front of the box but you have to remove all the screws that hold the box to the firewall. You'll need to pull it out as far as you can get it away from the firewall. The other thing thats a little tricky is getting the gasket behind the 4 little tabs at each hole. Once you remove the top half of the box you'll see what they look like. It will slip over the box from the front though.
I wouldnt use the 3M strip caulk in place of the gasket. That works better sealing the top half of the case to the bottom. The box has grooves for the caulk. Again, you'll see how it looks when its apart. The AC box itself has a wider channel for the gasket to fit in. You would have to use a ton of caulk.
The other thing you'll have to do is pull out the inner fender on the right side.