I am not following you, such as the statement 'For a 3% drop at 50' (100' total), the load is 74 amps, ".

Seems your making it harder than necessary.

Question is what can be max size breaker on #6 NM cable. #6 NM cable max rating is 60 Deg C column at 55 amps, allowed to upsize breaker rating to next standard size, which is 60.

The max continuous load on a 60 amp circuit is 48 amps, (80% of 60). I am not considereing this load at continuous, so 60 amps is the max.

If the load is at 60 amps at 120 volts for 50 feet, the Vd will be 2.9 %, just under the recommended 3% maximum Vd for a feeder.

Using the K factor is not the most accurate method of calculating voltage drop.

Use: 2 x Length x Amps x Ohms per 1000 foot of chosen wire / 1000 = Voltage drop