A bushing will reduce the mechanical advance. The only other way is to disassemble the distributor, then weld the slot, and custom grind it for the advance you want. That isn't easy to do. Some aftermarket distributors have adjustable mechanical advance. (MSD, Mallory, etc.)
It's a Dave Ray converted HEI, so it's already electronic, but I guess he didn't change the amount of built in advance. Got it. I'm going to try and machine my own set of different sized bushings since I can't seem to find a kit of them anywhere! I wonder if they are just press fit or threaded?
Dave's distributors are usually spot on. I think you are wasting your time. The bushings are ROUND and press fit. The slot is a fixed width on a stock distributor. That means the bushing can only be so wide before it binds in the slot. The MSD slot is wider, so it can take a variety of bushings to adjust the mechanical advance. If I remember correctly, you are trying to solve a detonation problem. If you cannot get anywhere by limiting your total advance, reducing your mechanical timing won't eliminate the problem. The engine may have too much compression to run pump gas.
Maybe this was covered way back, but if you are having detonation problems with only 26* total timing, your engine has some very serious issues! What is your cranking psi (WOT) ? Which plugs, what thermostat? If unsure how this stuff affects a detonation prone engine, please spend some time on Google. This is kind of frustrating to follow, watching one post at a time and never really addressing the root problems. Best of luck to you
I could be mistaken but I think this is the kit you need: http://m.ebay.com/itm/MR-GASKET-929...%3A60e65ba11590a6a8d6648f7effc467a4%7Ciid%3A1
