yep you gotta take the upper plenum off, been through this a couple of times when I swapped an 03 3400 in my old 96 Grand Prix. 150 dollars doesnt seem like a bad deal but I'd like to mention a few things...
Dont just replace 2 injectors, do all of them. I had good luck with injectors I pulled from the junkyard, from a lower mileage car. If you already have 2 going bad might a well get a set that will all flow similar and grab the whole fuel rail, just in case you lose or break any clips in the swap process. You'll need a fuel line disconnect tool, you can get a cheap set from a parts store.
Buy new injector o-rings, just good insurance.
The intake bolts take something like 9 foot pounds of torque, the intake will crack easily.
I do not know the mileage or maintenance of your car but consider replacing the uim gasket and see if the lower is leaking.
60degreev6.com is a good place to look if you want to do this yourself
"Going Off the Rails On the Swayzie Train"