How fast should a FB 40 be and how big?

im a high school junior and im about to be a senior this upcoming school year and im tired of playn the same position for 7 years so i started dropping weight i was 320lbs now im 240lbs 5:7 and i run a 40 in 5.43 can i be a fullback now or should i loose more weight n get faster or stay the same