Just got a Siamese for a 45-70 conversion project. Never messed with one before. I see no evidence of filing, grinding, etc on the feed rails; however, they appear to be too wide already. A loaded 45-70 will not stay in the magazine and the follower looks like it is riding too high. How would I be able to tell if this action has already been opened too wide?

I have one I had converted by E R Shaw many years ago. You can see on the right rail where they opened it up. My follower also looks a bit different than yours.

Try a different follower. Don't think I ever saw one like in
the picture of the 98.

I converted one to 45-70 in 1972 and the back of the rails needed zero work. The front of the rails only needed about .005 (.0025 per side) to pass the bigger rim.

Your follower is not a Siamese follower. It is NOT even close to the original in my rifle.

I did find two tricks were needed.

1. The original follower spring was old and very weak so I replaced it with a new 1903A3 spring. The new spring would lift even 500 grain bullets high enough to feed.

2. The 45-70 rim is so thick that it has a hard time riding up under the extractor claw. People grind away at the rails thinking they are the problem. I thinned the back side of the extractor claw about .005 so the 45-70 rim would feed up under it without binding when the round was at angle running up the feed ramp.

Those 2 things made it very easy to feed and I basically did almost nothing to the feed rails. Total time to remove about .002 from each rail was about 10 minutes and that included a number of checks.

Try thinning the extractor from the back side of the claw since it is a replaceable part. Your reciever is not replaceable.