Lets clarify:

You may push the knob in to make the overdrive available at any time. Before you leave the garage. Driving down the street. Whenever it suddenly occurs to you.

Anytime the green light comes on and it seems appropriate from 20 mph to 90 mph lift your foot off the gas and the trans will shift up.

Tromp it to the floor any time you wish for passing or hill climbing or embarrassing that clown in the 47 Caddy and it will shift down. Don't touch the freaking clutch.

If you are driving in the mountains and decide you don't want overdrive then merely do what the book says and push in the clutch and pull out the knob while the car is rolling below 60. The instant you move the knob the electrical circuit to the unit will be broken and there is no need to floor the gas. Think about it: if you floor the gas while holding down the clutch in any car you'd better have a magnet to pick up all the engine pieces.

When the knob is in the overdrive is always ready--tomorrow or next week. There is little need to mess with it.

The question remains if your unit works at all as the car is new to you. If when trying it out the car always seems to freewheel--it is not working. That is another issue to tackle.

You might have better luck finding an old Ford or Chevy seat from the period. Once its upholstered, no one will know or care.

Soldering the throttle arm onto the shaft is a way to take up play that sometime develops at the joint. The play is what keeps the carb from going to low idle--the arm hits the stop but the butterflies have not closed. That usually works pretty well and I have done it dozens of times in the last 35 years, many times right on the car. I would guess that your solder has jerked loose and you need to resolder or find another shaft and arm. Also good to check that the butterfly screws have not come loose in the last 69 years.

Personally I would just put a single 4 bbl on the car. It is far more enjoyable and and less headache causing. Then you can sell the dual setup to someone who is in need of the testosterone boost.

Industry standard convention is that positive camber means the wheels are tilted out at the top. Likewise, positive caster means that the king pin is tilted to the rear at its top--just like a fork on a bicycle.

All '56 crankshafts and 55 seniors are the same part number. I have a pretty darned nice one you could run with maybe a .001 or.002 bearing set on the center mains and the rest all standard.

I also have a NOS 55 Clipper 320 crank which is exactly the same except that you would need to have it rebalanced to account for the heavier '56 pistons.

And if push comes to shove I have a 374 all freshened up with new bearings timing chain, lifters, rings, and valve job. Please PM for details.

If you want something that will give you as much pleasure as having pushbutton shift in your car, might I suggest self sterilization with a dull butter knife.

As a mechanic I can not count how many hours have been subtracted out of my life by that device. Myself and others have had transmissions spoiled by it landing between gears. I've been on tours where a pushbutton car backs out of its parking space aaaaaand then has to be towed to a garage as nothing else will engage. My friend's sister had one brand new in '56; went shopping and then could not get out of the parking space--traded it in the next day. The stories go on and on.

Am very confused and need a picture. The exhaust valves are OK if they still have 1/16" of an edge at the top surface. I'm wondering if somehow your valve seats in the block are not concentric with the valve guide.

I have always ignored that practice and install the lifters with the kerosene in them. As the viscosity is so low the lifter will not hold a valve open off its seat and will leak down just like on the tester. The remainder is very quickly expelled and replaced with engine oil upon startup.

Yes, the valves must absolutely be above their seat when the valve springs are not installed. In fact they should be above their seat by about the same spec that you found with the checking gage.

I don't recall if a '39 Six has a vacuum advance. If it does, is it working? Is the mechanical advance working? Even without looking for the flywheel timing marks you can observe the function of the advances by means of some random chalk marks on your damper. They must march upward as you give gas up to 2000 rpm or so and retreat upon returning to idle. If it has a vacuum advance the marks must jump back partway the instant you snap the throttle shut and then return the rest of the way as the revs drop.

As far as initial timing goes, an expedient is to advance it til you notice the the engine fighting the starter and then back off a bit.