I have a Steve Baldwin Hi Wall in 40/65 which I only use BP loads in, however, if my supply of BP should run out I may have to use smokeless, which I have no experience with. 
I use three different hand cast bullets ranging from 360 gr to 410 gr, all  my shooting is done on targets at 100 m off the bench.
I rarely ever run out of BP but if I do I just need a backup plan.
Cheers
Mike.

A subscription to Loaddata.com or buy the latest edition of the Lyman manual that will give a lot of options that you can compare to what ever smokeless powder you have available

I go along with these guys. 4198 is a good powder in most straight wall case rounds. I also have the Snover bullet in a 40-70 and have had good luck with it.



JMH

Mike, just use AR 2207 as a substitute for 4198. I use it in my Meacham HW in .40/65 and Hepburn in .40/70 SS. No problems at all.
Spud

My 40-65 shoots well using 30.0 gr. of IMR 4895 and the Lyman 410663 410 gr. Snover bullet.

I realize the last response is about 6 months old and I am new to this site and I am just starting down the buffalo rifle rabbit hole. I found a Pedersoli 1874 Sharps repro in 40-65 and, not wanting to shoot BP, I asked around about smokeless powder loads and got answers all over the map so I began experimenting. I have settled on 35 grains of Varget and a 400 grain .410" dia. bullet from Bear Creek Supply. That load gives me a mean muzzle velocity of 1289.4 fps with SD = 16.21 and ES = 19. At 100 yards, using cross sticks and iron sights, I am able to consistently keep groups under 2". I am waiting for the weather to warm up so I can get out on the range and see what I can do out to 500 yards.

My Browning BPCR 40-65 shoots well at 150 meters using 21.5 gr. of AA-5744 and a 410 gr. cast bullet.