Has anyone ever used the imr-4227 or Sr4759 powder "trapdoor loads" in the 45-70?
My Hornady book recommends loads with these and both are pretty close in charges,,,
LGS is out of powder/everything and I have these already...
I should be able to substitute cci200 primer for a Remington 9 1/2?
A 300gr.HDY.HP &speeds around 1700 I should be good to go for anything south of the Mason Dixon line ?

I use a lot of SR4759 in the 45-70 with heavy bullets. However my rifle is much stronger than a trap door and I shoot at much lower velocities than 1700 FPS. My loads are about 1100 fps and are extremely accurate. I use both Remington 9 1/2 and Fed 210 primers a lot. I have used some WW LR primers. I don't think primer choice is very critical at these low pressures. The primers and the brass will take way more pressure than a trap door. It is your responsibility to work up to safe operating pressure.
I don't really consider the 45-70 a high velocity cartridge. I have shot loads that you describe at whitetails using a much stronger single shot. Mostly I consider the 45-70 a target and plinking round using cast lead bullets.

Are you shooting these in a tradoor or only using the load data for trapdoors?

For a Trapdoor, I don't typically like jacketed bullets, prefering istead, ca 405 hollowbase cast bullet. I use BP or BP substitute. I use dto shoot my carbine in cast bullet silhouette matches a lot.

In a Marlin either of those powders would work. I have used Unique in a ton of Cast Bullet loads.

NO I do not use those fast powders for light loads; too much room for error. I use 4198/3031 range of powders. Or black powder.

Oh, I should have mentioned that I use a lot of AA 5744; that works well. It is a bit slower than IMR 4227 and might be less dense. I guess I shouldn't be too hard on the 4227 range of powders; I have used faster ones like blue dot and do not do it any more.

I use 4227 and Unique all the time for cast loads in the 45-70 and 38-55. It is true they don't fill the case and could lead to a double charge situation, but with proper loading techniques and careful checking, you won't have a problem. I get excellent accuracy, mild recoil, and a lot of fun shooting these loads.