That Sunbird down haul sounds just like how mine came from previous owner, except the hole and block was for a 3/32" wire cable down haul and was located just in front of the molded-in cam cleat wedges. I often use a 3/8" dia x 24" aluminum rod to "get the CB started" when the DH purchase won't. I also used it to extricate the CB when it was stuck in the trunk a couple days ago. Unfortunately that required some thickened pox to repair a bit of the trailing edge. Also, 3/8" dia is too little width and can get caught between the CB and trunk side. The CB is about 7/8" thick (at the head) and the trunk about 1 1/8". My plastic pivot bushing was 1 1/16".
I found a new way to work on the CB without careening! As with careening I removed excess weight from the boat and then tied the anchor line to the mooring bit on the fore-deck then under tension to a horned cleat aft, around a sturdy tree, then to a horned cleat on the other side aft, then under tension to the fore-deck cleat (just trying to distribute the load here). Then I put an approximately 18" high uncrushable object (a well gusseted plastic toolbox in my case) under the stern. Then I let-go of about 8 feet of the bow eye winch cable and proceeded to drive away and pull the boat off the trailer- but only far enough to let the CB clear the trailer when it drops. Hopefully the under stern object stays put and the bow eye cable gets taught just right to have the boat hanging off the trailer yet still supported and able to work the centerboard controls. I also found that my 5ft wood step ladder, which has tapered sides, can be pulled under the CB to adjust the CB's height when needed. Also also, before you remove the pivot bolt make some marks on the CB and trunk opening (on the bottom) so you can get the pivot bolt and CB pivot hole/bushing all lined up, then use a marlinspike to fine tune the position and pop the bolt in.
The 7/16" double braid dia up-haul is working smoothly so far...