I really like AA5744. I use a RCBS .45-325-FN-U in my Browning 1886. I've been down below 2moa. However, I think the Shiloh will have a faster twist and will like a heavier bullet better. I'd call Shiloh and ask them what twist they're using in their .45 cal rifles these days and what weight bullet they recommend. I seem to think that when I was there 10 days ago, they said 1-16 for the .40 cals, and just don't remember what they were using in the .45s. I found that mine was made in '87 and has a 1-20 per the original .40-90SBN.
The .45-70 is a really easy cartridge to load for, and while each rifle will differ, my Browning likes a moderate crimp. Check out
Accurate Powder reloading data. IMO, I'd recommend staying at the low end of the recommended loadings for whatever bullet weight you choose. It appears that you have a fairly
short barrel, similar to ScottT's and may not gain either velocity or accuracy by increasing your charges. And regardless of the fact that you're using the same cartridge, and similar bullets, even with similar barrel lengths, the design of the action will have an impact on your velocity and accuracy. The very tight lockup of the Shiloh 1874, IMO provide a better basis for better accuracy, yet the attachment point of the forend can have a negative impact on same. The profile of the barrel, crispness of the sights, and just how you hang onto the rifle all affect the acheiveable accuracy.