I am thinking I want to shoot cast .45/70 bullets from my new 1886 lever. I don't want to cast my own so I will buy some...do I need gas checked ones or not ??

And don't gas checks make the cast bullets as hard on the barrel as Jacketed ??

Also what speeds do I run at to avoid leading ??

I don't think it would matter in the 45-70 as to which you shoot. The important thing is to find the most accurate load and stay with it whether they are gas checked or not. No, you will never wear out your barrel shooting it! Make sure you use a muzzle protecter when cleaning so the rod does not touch the bore and make sure you have a good rod, no cheap jointed or aluminum junk. Have fun!