At the risk of showing my ignorance because there are a lot (well there used to be) of people here who know much more than I, but here's a few questions.

I'm going to say by all the voltage checks you've done that you're way overthinking this. If you have spark at the plugs even if it turns over slow, it should eventually start. Another 6v jump might be nice to have about now. So assuming it DOES start, just doesn't fire right up, I would say you need to move on. So have you done some of the obvious things like looking at the plugs. Are they showing rich or lean or oily? Have you checked/adjusted the valves for proper clearances? If you haven't, be sure to really really bring the engine up to temperature. I recommend a nice lap or two around the neighborhood first. Have you checked and/or adjusted your timing via vacuum? I have had far better luck with a vacuum gauge than the timing marks. It doesn't usually happen but you might want to check the static voltage at the point with the engine off. Should be pretty close to battery voltage on one side and ground on the other. The ground side should show zero ohms to chassis ground. Both are assuming the points ARE NOT touching. If the gas is old, you may want to drain and replace. And if and when you do that, it might be a good time to inspect your tank. If you can get at the sending unit that's a real good peep hole. And if your gas gauge is bad, this would be a real good time to get it working properly. YouTube has some great videos on how to do that. And it may not need ethanol free but it might not hurt. 

My experience has been that slow cranking is the result of one, or a combination of, four things. Cables, connections, weak battery or weak starter. You advise you have heavy gauge cables. They should be at lease a 1 gauge (I use 0). "Clean" connections means every junction between the battery and the starter, not just clean battery posts. Every connection needs to be free of oil, rust, paint, etc.. (One member put star washers between his starter and bell housing thinking the points on the washers would dig into the steel and provide an excellent electrical path only to find they narrowed the electrical path to a few tiny contact points and greatly reduced current to the starter.) If all that is up to snuff and the battery tests good (batteries are sometimes bad from the factory so doing a load test on yours would be appropriate) if all that checks out good, that leaves the starter itself.

Hi Bob

I'm with Tiny on this.

Some of your numbers seem strange, especially in combination with each other.

For example, 5.85 at the coil when not cranking seems way low... but 4.9 while cranking doesn't.

So, you may have a combination of troubles, but Tiny's post covers about all of them, and I wouldn't rule any of them out.

By the way, how long has this been an issue?

Did it crank fine at one time?

Thanks all for the advice!  It did start much better this past summer. I have not tried today but at 37 degrees for a high I’m sure it would not start. I rebuilt a 49 Chevy truck about 50 years ago and I learned a hard lesson on good grounds and clean connections! The gas tank is clean, I replaced the sending unit. The gauge appears to be pretty close to correct. When it does run it sounds good. I have checked/adjusted the plugs, points and timing. The dwell meter reads aprx. 31 degrees. 
I guess I was surprised to see a volt drop thru the switch and a further drop out to the coil. Maybe the contacts inside the switch need cleaning? 
However, if 4.9 at the coil while cranking doesn’t seem low then that would eliminate a starter problem? Also that would point to a coil output or condenser problem causing my weak spark?

@robert-keck 

Not necessarily.  🙂

You may still have a weak starter.

A starter with poor brushes and/or a dirty commutator wouldn't draw your voltage down as it wouldn't be drawing any more amps than usual.

Coil and condenser will not of course affect cranking speed, but can certainly give you a weak red spark and poor starting.

Normally I would not recommend this approach but due to the low cost and number of headaches it can solve I do this.  I keep an extra coil, set of plugs, condenser points and rotor on the shelf.  These are easy to swap parts for ignition issues.  None of them would explain a slow crank though.  If the engine spins freely and there are no issues with the cables to the started I would start looking at the starter.  When was the last time you oiled the starter?  I found out the hard way that not oiling the bearings in the starter and generator can cause them to seize.  (lost my generator due to me not oiling it).

A low voltage reading in the ignition while cranking could indicate an excessive current draw by the starter. The voltage drop could be at least part of the overall problem.

Tony 

Voltage readings are acceptable. I would remove starter and clean with brake cleaner before going any further. Electrics like to be clean. You dont need to undercut commutator with correct solid copper brushes.Need to pass 200 amps so connections must be clean.