Yeah sorry. I think you might be SOL... Good news is if you want to be REALLY crafty you can trace the profile of the siding you are using into custom J blocks so no gaps... (scroll saw/bandsaw)
When I did our house I pretty much custom-made all the trim. (inside and outside) In a roundabout way it's a forever home so I didn't care. Turned out to be more a labor-of-love than anything else. (didn't spend a ton of money but spent A LOT of time making everything perfect)
Either way, think about the next guy. At my house someone (probably our kids) is going to be royally F'd someday. Everything is custom. Exterior rake boards, all J & H channel, boxes around receptacles, spigots, etc.... interior: all door & window molding, fireplace & mantel, etc. You cannot go to HD/Lowes and simply buy a new (whatever) if something breaks or styles/trends change. It's also the type of thing that "if you have to ask... you probably can't afford it". You can't just call a handyman/contractor and hire them to change out your moldings --or if our house ever needed a new window and someone broke the interior or exterior molding... they would be F'd... Time for caulk and paint.
I'm not saying don't use that as interior walls but think about the next guy with everything you do. The next guy will either be you, or he/she will curse you.
