So far the not so helpful AI generated answer is to use a 38-70 sizing die after anealing a 45-90 case then trim to length. If I had the correct die I would not need your useless help Google.

It appears that I will be able to avoid having to buy any 38-70 dies. A sizing die alone is $130 if it is even available from C&H. A 3 die set is $265. The Buffalo Arms brass is a perfect fit. I am able to only use bullets of about .377 or less due to a tight chamber. My .380 bullets that I use or my two 38-55s are too fat. A loaded round will not chamber.  I have a Hoch tapered bullet that works for fixed ammo when the case is expanded with a M die and I can slip fit them into a BP case when expanded with a custom expander plug. For those cases that I over expanded, running them into a 32-40 sizing die about 1/2” brings them back to specs. 
Next week I will be shooting BP and fixed with the Hoch bullet.

I think the .380 Saeco might be too fat for the bore. I haven’t tried my 248 gr bullets yet. If the Hoch shoots well I will not worry about other bullets.

The best that I could measure is:  The bore is .370 and the groove is just over .374.

I have lots of options and am curious as to how this will shoot. The BP loads I put together today are using a 30-1 Hoch bullet weighing in a a bit over 315gr  I will be breech seating the same bullet in 20-1. My other two 38s have mostly been shot using BP. One is relatively new to me using a barrel I bought a while back from Jerry Hartwig. There are not many matches around here any more with 2 at Tacoma that I help run. We are lucky to get 4 centerfire shooters. The match director wants to expand the .22 benchrest participation and will probably invite bolt guns next year.  The other match’s are CBA in Puyallup. Virtually no offhand or iron sights. I am considering joining the local BP ctg shooters at my home range.  Only 100 and 200 yds. I will do a shootout with my 38-55, the 38-70 and my seldom shot 40-65. To top things off I purchased a 32 cal blank and a set back 32-40 #4 weight barrel within the last week. I have 2 new 22s and two iron sight DST 32-40s to play with.  Too many choices. Too little time.