The single should be absolutely fine. I run a single, and I believe Drew does as well? Actually, mine is the stock one with 112k on it. I put about 35k on it with the cam, lift on my cam is .581 w/ XE-R lobes which have a harsh ramp rate. I run comp 918 single springs too.
You will need hardened pushrods.
Gaskets? Water pump is the only gasket I had to replace, the stock gaskets are re-usable. Again, stock gaskets @ 78k were 100% mint. The waterpump gasket was broke so I couldn’t re-use it.
Heads do not need to come off, only necessary if you are changing lifters.
No, they are the same pushrods. Base circle is smaller on the LS6 cam, but the valves are longer to make up for it and the springs are stronger to account for the lift difference in the Z06 cam.
Hardened pushrods are a must with a camshaft change.
Do some googling bro. Like with any platform you may have to go outside the usualy fanboy forum etc and see whats out there, some forums can be very biased. Not only that theres always a lot of shops in any platform that are like the unsung heroes and can do just as good if not better than some of the bigger guys