Will be using a commercial .408 nominal 400 grainer over IMR 4198.  Probably from a Magma mold but looks exactly  like the SAECO 40 cal 400 grainer. There is no Lyman Cast Bullet Handbook load for the 40-60 and my concern arises from the wide variance in acceptable loads and velocities  given in the LCBH for a close relative-- the 40-65  with a wide variety of nominal 400 grainers. Suggestions please

I shoot a Hepburn barreled to 40-60 Maynard for the singleshot class of our local cowboy lever match (200 meter). 
I use a different powder than you are asking about so this is not much help.

I load 20.5 grains of AA-5744 under a Paul Jones 406gr mini-groove (think money bullet) and it gives about 1175 FPS
Your right, no data. I started very low and tried too light and got sooty case necks and lousy accuracy. I moved up a grain at a time until accuracy was great and primers were still very round.

Black is not an option at this time.  I am up to 24 grains 4198 but the LCBH  has me worried about going higher as it gives vastly different pressure and velocity results for several different bullets all of same nominal weight in the similarly volumed 40-65 --cannot figure why.  I have 5744 --thanks for load-- and all the usual other cast bullet smokeless powders but 4198 seems to work best for me

Faster is not better for the larger cast bullets due to recoil.

Steve Garbe indicated there is more wind drift as you increase velocities from 1,100 fps up to 1,800 fps. 
He strongly advised at never exceeding 1,300 fps.

With my CPA .40-60 Maynard I shoot a 427 grain Paul Jones Creedmore at 1,200 fps with 49.7 grains of Swiss FFFg with a .060 vegetable wad.
She has a 16:1 twist barrel.
For distances over 500 yards I use 52.7 grains Swiss FFFg to get her up to 1,240 fps.

The loads for a .40-65 can be used in the .40-60 Maynard.
But DO NOT PLACE A WAD DOWN ON THE SMOKELESS POWDER LOADS.
Always start with a lighter load.

For a 400 grain lead bullet (From Lee and Hodgdon Reloading Manuals):
Accurate 2495: 33.3 (1437 fps) to 37.0 grains (1633 fps)
Accurate 5744: 20.7 (1200 fps) to 23.0 grains (1364 fps)
Accurate 8700: 46.8 (1041 fps) to 52.0c grains (1183 fps)
Varget:            35.0 (1438 fps) to 38.0 grains ( 1558 fps)
H4895:            33.0 (1423 fps) to 36.0 grains (1554 fps)
Benchmark:     29.0 (1321 fps) to 32.5 grains (1491 fps)
H322:              30.5 (1398 fps) to 33.0 grains (1520 fps)
H4198:            28.0 (1441 fps) to 31.0 grains (1572fps)
H4227:            20.0 (1277 fps) to 23.0 grains (1410 fps)

There was no listing for using IMR4198 in a .40-65 in their manual.

The failure at Quigley using Accurate 5744 was due to using a wad DOWN on the powder and using MORE THAN THE MAXIMUM POWDER LOAD for the bullet used.
Never take someone else's tested powder load unless you are using the same size of bullet.
Minimum and maximum powder loads are dependent on the size of bullet being fired.
Unfortunately some shooters use jacketed bullet information not understanding these loads CANNOT be used for cast lead bullets.