Take 4 or 5 Winchester 38-55's and reform them to Winchester 32-40

JWL
You can purchase Starline 38-55 brass by the piece from Buffalo Arms. Get the 2.125" brass to ensure your sized cases are long enough for your chamber. You need to anneal before forming in a 32-40 full length sizing die. To avoid ruining cases, size in 4-6 steps, redistributing the thin film of case lube between each step. I like Imperial but other case sizing lubes will suffice.
You also will need to make a chamber cast of your rifle so you can trim sized cases to perfectly fit your chamber.
If you have not formed brass before, buy a couple extra cases as you may wrinkle a case or two before you get the hang of it.
Feel free to PM me if you have specific questions.

Thank you for your input. I will give the forming from 38-55 a try.

JWL,

I just finished making 99, lost one in the process.

Starline 38-55 brass (2.125" version), standard RCBS Cowboy 32-40 full lenght sizing die, and imperial lube (wax not liquid).

Anneal the top 1/2 of the brass
Take at 5 steps to complete the process
Stop at 3rd step down, about 1/2 way down, and measure.
Trim to avoid crushing the mouth and neck on the last full length size stroke. 

Light lube each time.

Anneal again after last full length, trim again to 2.12"

You are done.  The good news is once you are done, the only way to screw up a case will be to step on it.   The pressures are very low and with good case management you will not need to do this very often.   

Take you time, you should not have many lost cases if you work them down in 5 sizing steps.  Do not try to reduce too much in one stroke.  The one I lost was from not enough lube, a 1/2" vertical v-crease down the case.   

HerrSchutz