Valve trouble, diagnostic help

[Ride The Snake!]

Hello everyone,

Wondering if anyone can shed some light on my situation. In advance, please forgive my lack of knowledge and experience, as I am truly a newb home mechanic. Done little jobs on my old car and this truck for a few years but never any motor work.

1970 GMC C1500 with some sort of stock 350 manufactured between '77-79, heads from '74, and 2g carb (no choke). I bought the thing for a rack and it had no history, and the PO kinda shrugged his shoulders about where the motor came from. But it ran strong as hell so I was sold. I always loved my dad's '71 'Burb so I really wanted onna these.

I developed a miss that seemed to happen over time. I realize now that the rebuilt carb I bought had a float height set to keep the fuel bowl too full, as wetness forms outside the housing through the upper gasket (but does not flow out lower through the throttle plate overflow holes). In short, I've been running a rich condition for awhile now I guess. Oops.

Did a vacuum test and had a consistent needle bounce over about 4 inches. 20 inches at idle, dropped to 0 on acceleration, bounced to 30 on rapid deceleration. Slowly accelerating did not steady the needle bounce. According to SecondChanceGarage, this means I have a sticking valve. Just glad it wasn't looking like blown rings from the rich condition.

After pulling the plugs, which all had some degree of carbon fouling, #6 looked particularly bad. I don't have enough of an eye/nose to see if there was oil present on the plug vs gas/carbon, but it had some wet, chunky black deposits. Valve cover gaskets were leaking oil on all the plug threads too--replacing those now.

Compression test read 150 psi on all cylinders save for #1 (160 psi??) and #6 (135). I forgot to block the throttle open during the test. I didn't perform a leakdown.

Yanked off the valve covers and cranked it, turns out #6 intake valve is barely moving. It does move, but not a whole lot. Wasn't sure what to make of it, so went through and adjusted the lash on all the valves as Chiltons describes for '72 and later motors. No change. Pulled the pushrod and it had some minor wear but wasn't bent; rolled on a flat surface well. This was my stopping point today.

Obviously, I need to go deeper. Should I be investigating the cam lobe and lifter for wear? My logic tells me it sounds weird for a single cam lobe to wear out by itself, but maybe lifters do? Or is this more likely a burnt valve? I would think if the valve was burnt or bent or whatever, there'd be some noticeably bad noise, and I can't hear any. And as stated, there is [I]some[I] degree of movement in the valve.

Just from my naked eye, it looks as if the rocker arm isn't being actuated fully by the pushrod. I'm hoping it really is just a worn lifter. But why would a single lifter wear out?