This might be long winded and not apply, but it seems like you might be where a friend once was. Been here before. Dont want to throw a grenade, but it could be a pain in the azz to get it right. After an experience years ago, I ALWAYS start on flat surface, level trailer and take plenty of measurements checking for a straight frame first followed by hardware measurements. Adjustments can be made with the bunks to correct if there are minor issues.
Soooo, buddy bought a boat {21' glass} on cribs followed by a trailer and stripped it. He soon discovered that the hull had a un upward sweep that was going to need your type bunks vs the ridgid and straight 4x4 type. He asked for help, but I really had no idea.
Off I went like a thief to the boat launch and watched for a simular boat to roll in. Asked permission to get measurements once they launched and got a baseline to start with, set the trailer up and loaded the boat onto it. We followed this by using any means at our disposal to lift the weight of the boat off the bunks by maintaining level conditions and making the required adjustments from there. Success in the end, but a bit of a workout. If you still have all your bunk supports on the trailer then you can skip to getting it on a flat surface for measurements. Do the work and make adjustments afterward. Hopefully someone can point to an easier way.
