Help with ordering my axle

I am not sure if this the correct place to post this question, but hope you can help me out.
I plan to order a FlexRide axle and am confused as to what size I need.
Outside of frame- 60 inches
Wall will be on outside of frame. 3/4 inch plywood plus .040 aluminum on each side.
The wheel that I plan to use are 14 or 15 by 6 inches wide
The tires I plan to use are P205- 70 - R14
What do I need to specify, so that I do have to cut my plywood for the arm?
Do I need a special length spindel to clear the arm?
Thank you for very much for any assistance or suggestions that you can provide.
Don
I plan to order a FlexRide axle and am confused as to what size I need.
Outside of frame- 60 inches
Wall will be on outside of frame. 3/4 inch plywood plus .040 aluminum on each side.
The wheel that I plan to use are 14 or 15 by 6 inches wide
The tires I plan to use are P205- 70 - R14
What do I need to specify, so that I do have to cut my plywood for the arm?
Do I need a special length spindel to clear the arm?
Thank you for very much for any assistance or suggestions that you can provide.
Don