saddlesore,
Mike Venturino has spoke about these issues with the 38-55 (I'm assuming its an 1894)?
If they chamber easy, they'll probably be safe, especially with your sedate loads. Another benefit is since it appears you are on the verge of easy chambering AND the bullet not slipping back into the fired brass (assuming it's not a ding from ejection), I'd tell you to slightly bell the case and seat the bullet again. If there's plenty of tension, not enough to push in the bullet deeper by hand, DO NOT re-size the case. Just bell, seat and crimp for your levergun. Hopefully the crimp on your seater die isn't scraping the front driving band.
Another question, the bullets that worked well measured .379 and were powder coated, correct? What diameter do they measure with your calipers or 1 inch mic (better)?
Coatings add a bit of diameter; it depends a bit whether the maker made .379 bullets then coated them or the coating was part of the diameter measurement. It might also be why they won't drop in a fired case. Never trust diameters put on commercially cast bullets; always try to verify, because it can eliminate easily avoidable variables.
I would try the same no size and chambering test with your .381 bullets; its pretty uncommon for a fixed chamber gun to have a smaller throat than groove diameter, unlike revolvers, where a soft alloy can help. So long as they chamber without forcing things or compromising function, the fatter the better, the harder the better. The fact .381 bullets are available are testimony to the common large bores (and probably throats) of some 38-55's.
