I recently built a box with spiral BLB. I placed them 6in OC also, am currently using 6, but only printing 8x10. I built the box much bigger so in the future I could make bigger prints and add more sockets with bulbs. My contact frame was about 3in from the bulbs, and I had no problems with light fall off.
I ordered my bulbs from Bulbman (LINK). They are 27w bulbs for $12.50 each, only a dollar more than the 20w.
I have been only doing some test prints for kallitypes to determine exposure times. The last test I did was for 2.5 minutes, and I believe I can still decrease the time.
If you have any questions feel free to ask.