Stock 1954 Mercury - To preface, I'm just an old-time shade tree mechanic and it doesn't take too much to stump me.  The engine has been progressively running rougher (missing) for the past several hundred (700-800) miles and last week I peeked inside the distributor cap to check for cracked or broken pieces.  Oh, the engine has about 2,000 miles since rebuild.  It appears that its firing well in advance, as at each brass electrode inside of the cap (each plug wire) the flat surface surrounding the electrode on the leading edge (counter-clockwise rotation) is blown or melted away in a tight vee pattern to a depth of about 3/32".  Timing is set at 3 degrees BTDC
at 500 rpm.  Also, the rotor cap electrode firing end is black (dusty carbon) only on the leading edge instead of the middle of its face.
The points, set at .015, appear to be opening correctly.

The engines starts immediately, idles a little rough, and then begins to protest when I let the clutch out.  What gives? ? ?

There's nothing that says the engine can't be converted back to the teapot carb and LOM distributor upon resale. Still, I "get" that you want to keep it original configuration and get it running right.

Did you get a manifold vacuum measurement with a gauge yet? This is a quick and easy way to determine basic internal mechanical engine health and setup. Compression, valve adjustment and ignition timing. Run the engine at a normal idle RPM. At Sea level you're looking for a steady needle at 18" to 20", if you snap the throttle it should drop to around 5" and rebound to 25" or so before returning to 18" to 20", this means the compression is reasonably good. It doesn't replace an actual compression test, but it's pretty accurate. And saves wear and tear on battery and starter and is a lot easier. There are several charts available for download on reading a vacuum gauge, the amount of info that can be gleaned is extensive in the hands of a competent mechanic.

Thank God and all you Y-Blockers, especially Tedster for yesterday's post.  I learned more today about a vacuum gauge than I have over the last 60 years of bending wrenches.  My old gauge, complete with original use instructions, led me to minor adjustments to the idle screws and to the timing.  Gauge read 18 - 20 at idle, down to 5 and up to 25 with a snap of the throttle, and back to 20 at idle.  Took it for a spin and didn't feel or hear any misses and came up the hill to the house in 2nd gear with no hiccups or hesitations.  Looks like I'm back on the road again!