Search Ebay for "235 Chevy vacuum advance". There are three of them listed this morning. They're fairly pricey, but scarce parts usually are. Be aware that you'll have to re-use the part of your original advance unit that clamps to the distributor housing and has the "octane selector" slot. There are two different shapes to the vacuum advance bracket available, but the only difference that will make is in the routing of the vacuum line. Either one will be sufficient to advance the timing properly.
While you have that plate removed, take some time to clean and lubricate the centrifugal advance system. That's the weights and springs under the plate which move the breaker cam forward as the engine speed increases. I use Lubriplate lithium base grease as a lube for the advance but a SMALL amount of high temperature wheel bearing grease will work OK. If you have a dial indicator, check the lift on each cam lobe. Most of the distributors I have checked have noticeable differences in the height of the lobes, which affects the timing between cylinders by changing the point gap. I remove the insulator in the side of the distributor housing and adapt the dial indicator plunger to bear directly on the cam. Good luck!
Jerry
