Single breaker? Not 220 to the new subpanel? Ground & neutral need to be seperated in a sub panel, not combined, with the ground bonded to the enclosure, the neutral isolated from the enclosure. I you want 220, you need a double pole breaker and 3 wire. See image below.
#8 is not enough for 50 amp, and may be marginal for 40 over 100'
PM sent.