Just in case this information is useful to anyone, thought I'd pass it along.
I started with a light load of IMR 4227. Light because I want one round to work not only in my Spencer, but potentially a 1873 or others in the future. The latest Lyman manual quite handily divides its 44-40 loads into 1873 safe, and hotter loads for more modern rifles.
I picked 4227 because I have it on hand for another oddball cartridge, .351 WSL. BTW 4227 has worked great in that caliber.
All rounds fired great, however left behind large chunks of burnt powder. Learned later 4227 is notoriously dirty, especially a lower pressures. Left enough residue in fact that after only a few rounds the Spencer action wouldn't lock up properly due to this residue. Loaded up some "max 1873" pressure loads, and got the same result.
Next gave a shot with some Alliant 2400. Reasoning was that if it worked, it would again be compatible with my .351 WSL, even though research showed might be just a dirty as 4227. Results turned out to be much worse. Best I can tell I was failing to get any ignition at all, with only the primer firing. This was surprisingly enough to have the bullet clear 30 inches of barrel. After the 1st shot was only a primer pop, made sure to inspect the bore with each shot. Both min and max "1873" level loads failed to ignite. This was rather surprising. I guess maybe a change in primer might be something worth looking at? But I'm not very keen to pursue this load any further after such dismal performance. Kind of surprised Lyman lists the load. Or maybe I'm missing something here? I'm sure I'll find good use for this stuff in my .351 WSL.
I also liked the idea of avoiding a pistol powder for that extra safety cushion of being unable to double a load of powder, but looks like I'll need to go the pistol powder route. Next stop is 5 grains of TiteGroup, which advertises itself as having very consistent ignition, clean burning, and not being position sensitive in the case. This all sounds great to me after my failed attempts. I'll report the results in case its useful to anyone here!