You have asked a question that is going to get many different answers. I tend to listen to what JD Jones (of SSK fame) has to say about the Contender in 45/70 . He says the Contender will take 28,000 cup loads as a good working safe maximum load.
If anyone on here knows more than Jones I am ready to listen provided they give evidence of their knowledge with actual experience.
That being said look in your loading books at data up to the 28,000 cup limits and use that.
With a 300 gr (light for caliber and case) H4198 and IMR 4198 with be better choices in the long haul. It will get just as good velocities with less powder.
Now 3031 is an excellent powder though and will serve you well since you have a good supply of it.
Remember use data up to 28,000 cup and you will be okay.