Last year I had my clutch, pressure plate & throwout bearing replaced. Within a few days I developed a "chatter" while shifting in traffic. I had the shop reinstall a new system and all was fine for awhile.

The hard way would be to remove the inspection plate and count the teeth on the flywheel.  Mark one with nail polish or a carpenters crayon or welders chalk, then turn the flywheel with a flat screwdriver wedged into the teeth up by the starter, using the starter for leverage.

I have absolutely no idea on the input shaft.

I'm sure one of the real gearheads will be on here after work and can answer.

Is it possible that you have a small leak of some kind that gets on the clutch plate during in traffic driving?

When you say in traffic, do you mean you sit with the clutch pressed in or are you kicking it out of gear and letting off the clutch?

Good luck!

Who is asking those questions? You should be able to get flywheel and even input shaft specs by visiting Advance auto parts. They can look up the parts and give you the info from their spec sheet. I love these guys.

Thanks guys, I spoke with a Centerforce clutch guru, he's the one who asked about measurements, he said it could be one of two sizes . He also said if a rebuilt clutch was used they tend to chatter a lot. As for the heat, these BB give off a lot of heat. I would think that if it were oil getting into the clutch, the mechanic who replaced it for the second time would have said something, or charged me.

I'd be looking at the surface finish of the flywheel. If it was "turned", or machined with the first clutch replacement, or even both times, the finish may be too rough. Happens ALL the time. I've seen flywheels come back from the machine shop that were just JUNK...so rough you could almost cut your fingers if you rub across the surface.
Also, make sure whoever is replacing these clutches replaces the pilot bushing in the crank, and checks the end of the input shaft on the trans for excessive wear. if the bushing/shaft is worn, it will cause a chatter.

I believe the clutch size is 10 1/2", with a fine splined input shaft.

Depends on the flywheel...is it the original? If so, yeah, I probably would. Do you know the history of the clutch, as far as how many times it has ever been done? It may have been machined too many times already.
On my '74, I know for a fact that I am the only one that has ever touched it after the guys in St. Louie put it together, so I know it's done right, and it's only been done once.
I would suggest you take it to a good machine shop, if possible, and get an opinion/measurement from them, as far as the flywheel. If the clutch has been chattering, it would be best to replace it, but...without actually seeing it, I couldn't tell ya for sure...

Great advice guys,

Replace everything and I mean everything.  Clutch, Pressure plate, Flywheel, input shaft bushing, clutch fork, clutch fork ball, throw out bearing, clutch fork boot and ensure the dust shield is installed when the job is complete.