I stand corrected. My calculation is slightly wrong with slight assumption. Have a look at the photo of micro quad with 5 inch prop with min 1inch gap between prop tip. Its diagonal is 13.5 inches and this quad will in no way fit a 330mm circle dia.

if I correct my calculations, to have min 1 inch gap between tips the motor to motor center forms a square of 6 inches. Diagonal of square formed will be (36+36=72) square root of 72= 8.48 inches. Add 5 inches to that for the two prop half's on either side makes it 13.48 inches= 13.48x25.4= 342mm. having gap less than that is not desirable.

my micro quad  multwii on 3S 1300mah flies for 12-15 min without any payload. To carry a APM setup without modification to frame and electronics is tall order that's reason for  my recommendation.

Sundaram Ji, as per my design, I should be able to fit four 5" props with 12mm gap between prop tips within 330mm.

Your quad seems to have been generously spaced out, our's is going to be tight.
15mins flight time is quite impressive. What motors are you using? Thrust and power consumption?