I have a backhalved 1969 Pro Street Camaro with a ladder bar setup. I didn't build this rear of this and am new to ladder bars (and suspension geometry!)

The vehicle does not sit quite square over the back axle. The (huge) rear tires have little clearance with the fenders, which I could live with if everything was even but one side has a reasonable gap, the other side has almost nothing and will rub against the fender.

I have tried varying the setup of the ladder bars to even the gap but can't seems to get it right. I have managed to get it square several times with on ramps but put it back on the road and it falls back into the wrong place. I have managed to move the gap from one side to the other but have no idea how.

The ladder bars have a diagonal rod between them which I undestand to be a track locator (watts link). I am not sure if it is this I need to adjust, something else, or if the rear was just setup incorrectly to start with. I find the latter unlikley as my existing ill infomred efforts have at least moved the gap around.

I am at a frustrated loss! Vehicle is setup for racing but is street driven on rare occasions.

I would start measuring everything on the rearend.
Distance from axle ends to where the ladder bars mount, cross measure (right axle to left ladder, etc). If all the numbers come up okay, start adjusting the Watts link.

The ladder bars have no bearing on the side-to-side gap. The pan hard bar (most likely what you have - consist of a single link from one side of the frame to the opposite side of the rear axle) or watts linkage (sort of works like a pan hard bar but is made of two links attached to both sides of the frame and to a link attached to the center of the rear axle) is where you need to concentrate your efforts. There should be an adjustment there that can be tweaked to center the axle. If properly built the adjustment is made just like a tie rod on the front end with a tube with LH and RH threads on either end, jamb nuts and screw-in rod ends. Loosen the jamb nuts and turn the connector link to move the rear end left or right 'til centered.

The diagonal link is not a "watts link". It is however your locating link and that is what should be adjusted for side to side clearance.

Disconnect the diagonal link at the front and then center the rear end. Adjust the diagonal link so it bolts back to the original location without moving anything and you should have the rear adjusted.

The diagonal link is pretty much a race setup. For the street most people use a Pan-hard bar or Watts Link, both of which are mounted between the rear end housing and the frame and pretty much hold the rear end in the center of the chassis as the suspension goes through its movement.

Centerline

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Five minutes of effort and I have a perfectly centered rear. Its amazing how easy these things are when you know how. Its also amazing how I couldn't figure this one out for myself. I was convinced it was to do with ladder bar length and thought the diagonal bar was just for triangulation.

Thanks! Seeing as you guys are so clever does anyone know what I can do about the ladder bar shaped bump on my head

Want to compare you ladder bar bump with my Willys oak soft top support crease across my skull?

(Super-stiff Willys Gasser leaf spring suspension + oak wood beam across the soft top insert that is right over my head + rail road tracks @ 55mph = crease!)

Ouch - I feel your pain! I could argue that oak is softer than steel but that would just be pedantic. Oak is quite hard enough thank you. Hmm. Carved oak ladder bars. Now that might look really cool.