Rough idle turned to crank then ka-thunk
I never noticed a plastic timing gear in my boredom searching I've done so that a new one for me
Just to be on the same page, yes the nylon timing gear on the bottom of the Distributor shaft is known to crack, thru one of the Roll-pin holes. I've taken quite a few Ds out that were cracked in this way. It seems that when they get old, they get brittle.
As to timing chains and slantys;
I have never personally rebuilt a slanty. I usually just replace them with 318s lol. However, I have had in my lifetime, replaced two slantys with rebuilt units, for the specific purposes of increasing fuel economy. Unfortunately, I must never have been any good at tuning slantys, cuz all my tuned 318s got better hiway fuel-economy than even the best of my slantys, lol. Altho, there was this one hi-compression slanty that I had built for me, that could have maybe been a runner, had I tried harder........