Looks like you are off to a good start. If you have a Harbor Freight nearby you can pick up a cheap dial indicator and magnetic base. They aren't of great quality but will do for this. Oh, and don't bend the snap ring back into shape just yet...do that when you reinstall it. It's not a springy snap ring, it bends, and you're technically only supposed to use it once, but since a new one costs $30 or so, I think twice will be ok as long as it doesn't break when you reinstall it.
A lot of people use button-head allen screws to attach the ring gear to the carrier, but I don't think that's a great idea because no one seems to make those bolts that have an unthreaded portion of the shank, which is what you want to be transferring the load between the ring gear and the carrier. If the threaded portion of the fastener is bearing the load, it'll squash the threads or gall up the holes in the ring gear and carrier, depending on the hardness of the bolts relative to the ring gear and carrier. Now it may be that the bolts provide sufficient clamping force to allow the friction between the ring gear and carrier to transmit the torque, but I did some calculations with some educated guesses for some of the numbers I don't know (I don't know the diameter of the bolt circle for the connection between the ring gear and carrier, or the contact area between the two) and it looks like it could go either way. I'm also assuming that the full torque of the engine can be put to the ground (it can't, or at least not without something else breaking first) so it might be ok but I just don't know. Anyway, where I'm going with all of that is, if I were doing it again (or what I would have done if I had been able to use the CI ring and pinion when I did mine) is use aircraft-grade hex-head bolts, and here is why:
1. They come in many different sizes and grip lengths, so you can get ones that have an unthreaded shank length (grip length) that comes close to the thickness of the ring gear and carrier you are bolting together, so if the bolts don't clamp the ring gear to the carrier tightly enough, the bolts will carry the load without galling the holes in either the ring gear or carrier, and
2. They can be torqued enough to give you some chance of clamping the two pieces together tightly enough so the load is carried by the friction between the ring gear and carrier.
So basically, use those, and you'll be safe either way. I hope I didn't lose everybody in that, but I just wanted to explain why I'd do it that way. BTW, the bolts can be obtained
here. I can't remember what size the rivets are in there; I think they're 5/16". If that's the case you need the AN5 bolts I linked to. If they are 1/4" you need AN4. You can use those bolts with ordinary fine-thread metal locknuts. The numbering system for those bolts is kinda confusing if you aren't used to it, so if you tell me what the hole diameter actually is, and the combined thickness of the ring gear and carrier where the bolts go, I can tell you exactly which ones you'd need. They are a little bit more expensive than normal hardware-store bolts, but you will absolutely not have any issues with them if you choose to use them.
One other thing I should mention is that regardless of which bolts you choose to use, two ways to lock the fastener together is a good idea, i.e. locknut and lockwasher, locknut and loc-tite, etc. so that if one locking method is compromised for some reason the bolt will still hold.