BrettV, welcome to the world of double rifles. I suspect your rifle will shoot acceptably with some type of factory load, which it was likely regulated with. Seeking good gunsmith advice is always a good idea, but even that gunsmith would be able to tell you more if you have tried a few things first. Dewald gave you some good suggestions. If the regulating load is marked on the barrels, try that. If not, you should realize there are at least four distinct types of .45-70 ammunition commonly found on the American market, and each is quite different. Try some standard 300 gr. ammo, some standard 405 grain ammo "for all rifles", and some +power 405 grain ammo " for strong action modern rifles". If that doesn't work, try some Hornady Leverevolution ammo.
When firing your double, don't shoot it as you would a bolt action - off a bench with both front and rear bags. Shoot while standing and support the rifle under the hand that holds the fore end only. Grasp the fore end firmly. A double is quite a different beast than a bolt action, and requires a slightly different shooting technique.