Yes, I HIGHLY recommend removing the manifold to replace the GP's, especially the job hasn't been done in the last 10 years or so. As far as 6mm hex sockets go, not all are created equal. Try a 1/4" drive 6mm hex socket, it will give more clearance when removing the fasteners from the lower bolts. Additionally, a magnetic pickup tool will be useful if you drop a bolt into a nook and crannie that your finger can't reach into.

Things you can easily do with the manifold removed:

1. Replace fuel injector line clips and rubber bushings
2. Replace intake manifold gasket
3. Clean out intake manifold of crud/carbon/sludge
4. Lube throttle linkage
5. Adjust ALDA
6. Replace shut off valve