Every couple of years one of the gun mags will do an article on shortening your barrel. It is pretty easy if you are mechanically inclined. The biggest problem is everyone likes to think they are mechanically inclined and it is not always true.
Check amazon (or B&N, or whoever) for wolfe publishing's book on "gunsmithing tips & projects" (or any similar book would work). There is a reprint of just such an article in this book.
I think you are looking at $50 to $75 for the tools to do this yourself. You may have many already. If you do not already own some of the tools you need, and use them often, you might want to skip doing it yourself.
A gunsmith might charge $100 to $125 for this type of job, not counting any stock work. The trick is to find a good gunsmith and one who charges reasonable rates. Let us know how that turns out.
Getting a sight on the shorter barrel is the toughest part I think, and you have to watch the point of impact vs aim with the shorter barrel (needs a different height on the front sight). There are a lot of options here, from custom ramp sights to a clever re-use of the original sight. JB Weld or Acraglass can do this job as well as welding or brazing. Or try a dovetail install.
I concur with the carbine thoughts, but M38 are rare these days and even a nice M44 is double the cost of a 91/30. Also, you already have the 91/30. Selling/trading might be a tough road due to lack of interest. Also, you may be looking for a barrel length between the 91/30 and a 38 or 44.
I am guessing that you want a shorter length for some specific reason, but I tend to agree with the other posts that the 91/30 is a really good length and handles nicely as-is. Maybe live with it for another couple months and shoot it some more, then re-evaluate.
Lots of folks will come unglued at the sound of you powering up a hacksaw next to a military collectible. I'll let them fight that fight. Use google (or similar) to find tons of pictures on the internet of various weird hacksaw customized rifles that look like utter crap. Also check prices for last decade's cheap military surplus rifles and compare unaltered vs. hacksaw values.