Leading won't be an issue if the proper diameter cast bullet is used (largest diameter the throat will allow).
That ^^^ I loaded .459" cast bullets when I had my 1895 and got decent accuracy, I still load the same for my single-shot Pedersoli. I don't cast... I get my bullets from Oregon Trail or Montana Bullet. Depending on how much he plans to shoot, you might be better off just buying 500 bullets rather than tooling up for casting, unless you just want to.
Faster powders are better for lower velocity loads... IMR4198 is my favorite; a good substitute would be AA5744, and IMR3031 will work as well. Once you go past that (IMR4895 and slower) you are going to see more velocity... but more recoil. For sure... and even with 350grn bullets, etc.
My standard load is any 350grn-405grn bullet over IMR4198, target velocity is 1300-1500fps and is good to 700yds... so far.
I'm not a big fan of pistol powders in the big .45-70 case, I have witnessed a case blowout with 15grn Unique under a 405grn cast bullet... a classic loading. It's just a factor I don't have to fool with, so I don't. I have loaded it, and it certainly works, but, again, I don't have to. TrailBoss might be a caveat... it's bulky to help soak up some of that case volume, but I have no direct experience with it. 2400 might be another.