Been there, done that - you can make it work - and well.  Just use a 500+gr bullet (520 - 530 money style) with tapered driving bands to maximize powder capacity (think a Buffalo arms mould).  That way you can get 70-80grs of powder in the case, which should be more than good enough.

My own view would be to go to a 45/90 with a PP smooth sided bullet. It wouldn't have as much as much of a rainbow trajectory as the 45/70. Grooved bullets cause aerodynamic drag which slows them down faster than smooth sided bullets at long range. For lubrication a grease cookie between two card discs below the bullet is the standard way of lubing a PP bullet.
A long 3 to 4 foot drop tube will settle a powder charge better than compressing it, the longer the tube the better the powder settlement. In most cases in long range shooting a PP bullet is seated no more than 1/4" into the case. Those Long Range shooters who compete and do better in such competitions use a 45/90, it has more oomph, than the lighter charged 45/70.

In the end it's a case of what can your shoulder stand in the way of recoil. A 45/70 round will get to a 1000 yards but it's moving like a geriatric at the end of a Marathon Road Race when it gets there. The 45/90 has more authority at the far end of the range. That's just my opinion of course, YMMV.

Harry

Bore riding bullet, step down basically. 556 gr bullet.

Able to load as a 45-90, works very well.

Bullet is not seated all the way down in the case, no need to, the step down keeps the bullet straight with the bore.

The money bullet at about 535 gr seems to be the bullet of choice. If you get one with the first one or 2 driving bands reduced you should be able to get close to 70 gr of powder in your case, maybe a bit more. I think that last years 1000 yd match at Read Range was won by a 45-70. 
Sam

HANK:
   The old-timers quickly went to cartridges that gave more velocity for their LR shooting.   IMHO, this was owing to vertical stringing from velocity dispersion.   They had no satisfactory way of measuring the velocity dispersion, so could not refine their loading techniques to minimize it.   The vertical stringing depends approximately inversely on the square of the velocity dispersion, so even a 10% increase in velocity would show about a 20% decrease in vertical dispersion.   
    It follows from this that since we can measure velocity dispersion (expressed as SD or ES) quite readily, that this provides a valuable tool for load development.   If you can get the velocity SD down into low single digits, you don't have to worry much about the vertical stringing.  AND you will be getting less wind drift than the HV guys.   Study the ballistics tables.
    A fly in all this ointment is that many ranges have specifications for minimum performance for LR cartridges, because they worry about bullets coming into the targets at too steep an angle.   Make sure that your bullet weight and velocity come up to the performance specifications required at the event you contemplate entering.

CHRIS
RGChristensen

Quote:

Duplex loads from the NRA BPTR rules (both mid-range & long-range).
3.17 Ammunition - Ammunition may be fixed, breech seated or muzzle loaded. Only commercially manufactured sporting grade black powder or Pyrodex may be used for the entire powder charge in muzzle loading rifles. Breech-loaded ammunition only may be made of black powder or Pyrodex or a combination of black powder and up to 20% smokeless powder by weight. WHRSA requires the use of black powder only.

Is breech seating allowed in 1,000 yd  shooting and is there a time restriction? Thanks for your answer, Hank45