I know this is the race section, but....

I have 1979 D-100 truck with a 360. Cruises at 18-20 inches of
vaccuum on the freeway at about 2800RPM at 65mph, level road.
Motor is very mild and fairly new, in good shape. Nice truck.

But, I'm only getting 9-10 MPG. At that manifold vaccuum, you
would think there's not a lot of parasitic drag, binding, etc.
Also, coolant temp is stable and not out of hand.

The timing marks on the dampener are very hard to read, and,
I do not know for SURE if the TDC mark is really at piston TDC.
I realize I do need to nail this down...

I can't get a handle on the ignition timing. It has an MSD-6A
and some kind of Mopar distributor with vaccuum advance.
I'm wondering if I should buy a second distributor and have
a shop set up a specific advance curve, vaccuum + centrifugal,
something known at least.

If I buy another distributor, what's a good quality one ?
Maybe the MSD #8388 ? Another Mopar piece ?

I know I should look at the carb too (625 Edelbrock), but it
is in good shape and not weirdly jetted or anything. Really
there are no odd drivability issues.

No issue on the "race" section (they're pretty loose on that). I'm wondering if your gears are a bit low for highway driving (do you do alot of highway driving?) but changing gears ain't practical. Keep your present dist & tune it yourself. YES you need to find the timing marks/change the timing curve. What is keeping you from accessing the timing marks? You might even need to make a new pointer on the other side or underneath the dampener at 6 0'clock which you would need to get a $10 piston stop or make one out of a spark plug for that. Quick & dirty on the timing: set the initial to 20 & limit the total to 35 (with vac adv capped just for these 2 settings) with the FBO $22 plate (by far the best bang for the buck for this hands down). If you ain't pinging you are OK on the high end. basic compression test wouldn't hurt but I think you are OK there & dampener slipping does happen but is rare. Short version: bump & alter timing/strip kit for the carb. Holler how it goes & we'll continue on further.

FBO guy is really really good--will set it up so you drop in set it forget it