One thing to keep in mind is burn rate for the chosen bullet weight.  IMR 4064 is great with the 165 and even 180 grain bullets, but burns too fast for a heavier bullet like the 220 grain.  The powder would be spent before the bullet is 3/4 down the barrel, which means that 1/4 of the barrel length, the bullet is slowing before it reaches the muzzle.  Likewise, a slower burning powder like 4350 with a lighter bullet, the powder is still burning when the bullet leaves the barrel, which results in poor accuracy and performance.  I don't know the burn rate for 748, but if it performs well in 30-30 with a lighter bullet, then it should be ok for a lighter bullet in the Krag also.  I'll take a look at my Hornady and Lee charts to see if 748 is listed, and if so, I'll post the data for you.

Paul