I've got an 847, too. I disassembled the head completely during my rebuild, including that gearbox. The grease in mine was so hard that I had to chip it off the gears! Steve's video series was very useful, even though he didn't tear the guts out of the head. I actually worked out a deal to get some parts from his donor mill. He's great to deal with.
Anyway, the gearbox disassembly went smoothly. I just took my time and was careful not to force anything. I also kept everything (shims, gears, bearings) in the correct order. You should be able to do this. Mine went back together just fine.
The spring is a bit tricky to get engaged in the feed handle coupling. That caused me the most headache. You should make sure your spring assembly is intact and that the end is engaged. If so, all should be fine.
As mentioned, Well-Index still has a good many new parts. I'd guess the spring is one. They have been super helpful to me so far.
Good luck!
Doc