Ok, it's not a muscle car.  But I LOVE my pickup(s).  In this case, a 1978 Scottsdale K10 w 350 & AT.  The engine, transmission & transfer case were rebuilt before I bought this pickup.  Rebuilder was an SAE-certified mechanic w 30 years' experience.  He moved away & I can't find him.
I'm a passable mechanic - been doing this since I was 13 & I'm a grandmother now, but I can't find a solution.
Two shops have tried to find the problem I now have - it comes & goes - now long-term luck.  It idles rough.  It idles much rougher when engine is warm than when it's cold.  As in, I drive it 1-1.5 miles & that's warm enough that not only does it idle rough, it often dies when I leave it in gear at a light or stop sign.  If I put it in N, it doesn't die & the idle is faster.
One shop said the vac adv is fine; one said it wasn't working. When I tested it, it was working.  Found a vacuum leak in a short hose on the (Holley 600 cfm) carb; replaced the hose & plugged it - fine for awhile.
(I hate Holley carbs so am rebuilding an Edelbrock 600 cfm for this engine.  Have bone spurs in the L hip so someone else will probably install it.  Laying on a bone spur is murder..)
Replaced the wires.  Replaced the spark plugs.  Cleaned out the carb (several times).- never found any dirt but figured it couldn't hurt. Fuel filter is inline now, clear, fine, new. Sprayed carb cleaner 'everywhere' to try to find a vacuum leak - zip.  Gas mileage is horrendous. Pickup is pitiful when it's doing the rough idle dance; fine the rest of the time.
Backfires on occasion - have burned all 3 grades of good-quality gasoline - no change.  Replaced HEI distributor cap, rotor, & ign coil - small improvement.  Checked timing - book says 8 degrees BTDC but it won't run at all if I take it there, so right now it's at about 4.  I finally gave up & timed it by ear - when it ran acceptably, I tightened it down & left it there. Then took half an hour to quit gasping at the bone spur pain.. & took it for a drive. Fine for a week or so, then back to the idle-&-sometimes-die problem.
Anyone have any suggestions?  It's a fun truck when it's running right.. And what little sanity I had is leaving fast.  My grandkids are remarking on it! Thanks for any help..