New clutch chatter

[Chevy nutcase]

Finally taking the last steps of a journey started in October of last year. I removed the 250 and sm420 out of my 66 c10 to make room for a freshened up 350 backed by a new tremec tkx. While I was at it I replaced a lot of metal including both sides of the inner and outer rockers, lower portion of all 4 pillars, cab supports and parts of the floor but that's not why I'm posting this. Last Saturday the engine fired up great and the break in went without a hitch. The past week I've been doing exhaust, wiring, cooling and interior reassembly and today was the first attempt to move it under its new power. When releasing the clutch in reverse and moreso in 1st I felt a moderate shudder during the clutch initial bite that goes away once the clutch is fully engaged and im off the pedal. I thought maybe I forgot to cinch down the carrier or tranny mounts but all were solid. I went so far as to mount my phone under the truck using an octopus tripod to see if I can spot the issue and all I saw was the driveshaft jittering rotationally. I could take a better video and upload it to YouTube if anyone would like to see it. Between then and now I've Googled the issue and some suggest broken mounts or warped flywheel. Mounts are new and the flywheel was resurfaced. I even got offset bellhousing dowels and got it within .001 of the crank centerline. Some say it's not uncommon for new clutches and may diminish as it wears in. I'd really like ro hear your opinions having real world knowledge. Attached is the clutch I used from summit.

I posted this sametopic in "engine and drivetrain". Wonder which will be deleted by a mod

[SpudWrench]

Dont know if this is your case. but when i rebuilt my sm420 it took my quite a bit of work to find the correct throwout bearing as the one in my clutch kit was shorter than the original. The one in your picture looks like the one that was incorrect for me