If I were to build a rifle and used a chamber with dimensions different from what is considered standard, would I need a different reamer for making the sizing die than the one I used for the chamber?  This would be a case with straight neck for about .550" and then a straight  taper to the base.  Could this all be done with a finishing reamer?  Thanks

Given that it's a straight taper, rimmed case, you can make a sizing die by reaming it with the finish reamer, but holding it short of full depth.  Distance depends on the amount of taper.  

Precede this by reaming the die all the way through with a straight chucking reamer having the diameter of your case neck less some allowance for brass springback.  If you get this just right, your cases will not need expanding later.  

Such a die will create a case with straight neck longer than your chamber's, but the case will blow out to the proper taper on the first firing.  

Last year I made a perfectly serviceable .28-30-120 Ideal die this way.   Since my die will be used very little, I made it from 12L14 bar, which could be case-hardened, but I don't plan to.   

Obviously this doesn't work for any sort of bottleneck case, (it will work the brass in the shoulder are too much), and definitely can't work for any sort of rimless case.