Vibration in engine- help!

OK, big block 4 speed, runs great, recently put in new gauges including oil pressure. Drove it to Vermont Thursday night, about a 40 minute drive one way.
I was about 5 minutes from home and it started to run like crap. It felt like it was not firing a cylinder or two, it was sputtering a little and vibrating- which were both regular about every 30 seconds.
I got to the light before my house and when I pulled away the clutch seemed to engage later than usual, at the end of the pedal travel.
I had a similar experience and it was the cap and rotor, so I ordered a new one. Yesterday I got out the sand paper and cleaned both.
No more sputtering, but it is still vibrating every 30 seconds, enough to vibrate the car as well.

I inspected the clutch, and nothing looks out of whack, I ran it without the water pump hooked up to see if that was it, no suck luck.

I do not hear any knocking or noise. I stuck a long screwdriver to my ear at various places.

I checked the motor and tranny mounts and they appear to be ok, but can't they break inside and cause vibration?

I drained the oil and checked for metal shavings, and it is clean and free of metal.

If I hold the clutch pedal down the vibration does not change.

This is the second time I've extended the life of the cap and rotor with sandpaper, so I am hoping a new one will get rid of this vibration, but I am not holding my breath.

What else can I check?
My fear is a spun bearing, but I am not sure of the symptoms of that, if I can even get the pan off with the motor in, and how to visibly tell that is the case.

Pics of the mounts below. The driver one looks offset, but it looks like it was made that way, the passenger one looks good as well, just a little slanted. I can't get a good pic of the tranny mount, but it looks fine as well.