Whatever the original ship had for distance between barrels is typical. Most look pretty close to scale.
I put the breach nuts as close as I can and still get a wrench in there to tweak them. On trips I put the center one slightly higher, like 1/16" and toe the outside ones in 2deg. That way at 12" off the stern they hit in a dime sized triangle. Hopefully taking all the balsa apart inbetween them.
