That's a typical modern lead at what appears to be 45 degrees. I have two in .40-50SS I shoot, and one is a modern GM barreled Rolling Block, the other a Schoyen barreled forged Ballard. The Schoyen has a long lead of around 11 degrees, but I shoot both using my RCBS CSA 370 gr. cast bullet. 
There's no reason to need more case volume if you load it with smokeless, but might need more if you shoot it with BP. I use 4198 powder and there's plenty of space for a good load. I think if smokeless loads had been around before the .40-50SS became out of production it might have stayed around a lot longer. It really shines with smokeless in the short case.
I have a bunch of rifles in .40-70 and love them, but the .40-50 with smokeless is just as much fun, and the smaller case makes it more efficient.

Ronnie, with the 18 twist a 400 grain might not hold up at distance. I tried that with my 40-60 Maynard and it didn't. Consulting with Dan Theodore he said with an 18 twist better try the 370 grain Saeco and it got me 30 animals first time out. How much BP compressed could you get in your brass with a 370 grain seated as far out as you can? You may not need much in that smaller case.