Just had to add one more thing. I rewired the trailer yesterday with LED lights, and noticed how pathetically lopsided and uneven my CE Smith boat guides were. In all honesty, I threw them on in a hurry after I purchased them, but when I was tackling the rewiring job and had the boat off the trailer, I got a close up look at how poorly of a job I did on the initial install.
I loosened both brackets and removed the guides, and then measured my Jon boat at it widest point (duh). Then I purchased a 4' feet of angle aluminum from Rural King for fifteen bucks (same type that I used for the front deck support) and positioned that thru the boat guide brackets. I then took careful measurements and marked each side guide so that they were centered on the angle iron. I then ran SS self tapping screws through the angle iron to the side guides before tightening everything down snug. Now the side guides sit at a perfect 90 degree angle to the trailer, plus the angle iron adds a good degree of rigidity that wasn't there before.
I have seen a few guys that used a piece of PVC pipe to tie the two ends of the brackets together, but the aluminum angle iron is much better IMHO since it passes through both guide brackets.