It would be best if they could have some adjustment. The way that the hull flares up and out from the keel means that they would be a little higher the further out from center they are mounted. Also do you have a solid flat "keel board" running down the center, on top of the cross members? This would be a great place to start. With adjustable uprights below your bunks, what many people do.....is have a boat lift set the boat gently on the keel board.. and then snug the adjustable bunks up under the hull while the boat sling straps hold it steady.
One other thing that comes into play is if the trailers hardware like the tongue winch and bow stop are in the correct position to promote a safe tongue weight? I have noticed that looking from the side, my 23 balances the trailer best if the front/to/rear centerline of the keel is approx. 9 to 12 inches forward of an imaginary midpoint between the tandem axles. Positioning the boat in this area gives a couple hundred pounds of tongue weight. With too little, the trailer tows squirrelly. This is also effected by how much tongue length the trailer carries.
SO, if you have a keel board, I could give you some range of measurements...that could key off of it, if you wanted but still, I think you would want the uprights adjustable.
P.S. your best next investment would be in fresh trailer brakes. Best of luck.