Hello BitOfShush,
Here is what I would do if i where you:
I would get a few 357 remington maximum brass , maybe from different batch or brand, shot ones even better.Let say 2 or 3 of each.
Get a set of reloding dies in 30/221 like redding ones.
Trim them close to the lenght you would like to achieve
Full length resize the cases above, but with your olive assembly removed.
Then re expend the neck with a tapered olive or an expender.
Trim to the desired lenght that you want.
Send the cases to Pacific tool and gage so that they can measure them and grind a reamer for you. The chamber cut with that reamer will be slightly larger than the largest case of the batch, and you know that what ever case you will resize with these dies will work in that chamber.
As to changing the dimensions of taper, of the the body or shoulder, will involve getting some custom made reloading dies, which can be done but pushed the price up.
See what the other come up with...
But it is still an interesting project, and those martinis are so cute
Nick