30-40 Trim die and a Hacksaw.
Then square up w. the trimmer.
That won't work to make .300 Savage length brass.
.30-40 Krag is longer than the .308 Win, it won't even stick out past the end of a .30-40 trim die. Is .30-40 possibly a typo??
A powered trimmer will be faster than a hand cranked one, but be aware there are times when power and speed are not your best friend.
You're not taking off even a quarter inch. Slow and even tedious can be better than fast and powered if your adjustment setting isn't perfect, or slips.
I used a Forster trimmer for a long time, but learned (eventually) that the small set screws holding the adjustment in place could slip if I was really cranking on the handle. Trimmed some brass too short, as a result of that.
I went to the Lyman trimmer a couple decades ago, might be the trimmer, or might be me, but I haven't had any problems with it.
.300 Savage brass is out there, lots of it, its been around a long time, and isn't going away any time soon. Might not be easily available as new brass TODAY, though.
How many cases do you need? In the old days, I'd tell you to go to the gun shows and hunt for once fired brass there. These days, that option is closed for a lot of us. If there are still shows where you live, they're your best bet, if there's nothing to be found on the interweb...
Posting want ads at your local gun clubs and gunshops isn't a bad idea, either.
Do be aware that if you form your brass from something else, internal case capacity can be different and loads need to be carefully worked up in the converted brass.