I have an original Highwall, re-bored to 38-55. The bore is exactly .376. I use RCBS .375 Win dies (Same round, only shorter, and loaded with lighter "express" bullets) I have found that LEE dies are a much better value than most folks give them credit for. (They are the BEST dies, bar none, for .44-40!)
Yes, I have used 30-30 cases. What I did, after looking at the 38-55 chapter in Ken Water's PET LOADS, was fire-form them rather than try and use an expander die, Water's suggestion was 15 grains of BULLSEYE topped by wadded paper towel. They result is only slightly shorter than a factory 38-55 case. Some cases looked a bit irregular around the mouth, but most were OK. With the lever gun, you might have to crimp. Better to hold out for some factory correct brass. Just modify enough cases to make some noise with your new toy. When I did mine, all I could get was old verdigris covered Dominion ammo, or "John Wayne " .32-40 commemorative stuff. I also tried .375 Win. CASES but I don't crimp in the single-shot.
POST MODIFIED 29 MAY 06; - DO NOT EVER TRY .375 WIN AMMO OR LOADS IN A .38-55. It is even a good idea to keep the two entirely separate as they resemble each other too closely, but have vastly diffenent pressures
Be careful about Overall length in the 94. (2.55inch NO MORE!) Get them too long and you will learn more than you need about disassembly of the magazine!
Loads are available in most older references. The Ken Waters chapter in PET LOADS is great, as is anything written by Mike Venturino. I use the SPG book for most BPCR.
If your gun is an older one, get the headspace checked, as some early smokeless HI -SPEED rounds were quite hot.