After the shifter is thru the gate, and the the other levers are properly detented in Neutral;;
There are three possibilities;
1) a failure to depress the clutch pedal far enough, or
2) a dragging clutch, or
3) a faulty interlock system
As for #1
push the seat closer to the pedal and try again. If your foot ends up on the floor, then you have the WRONG CLUTCH-PEDAL in your car. The six cylinder pedal has a different ratio than the V8 one. This is due to the two different sized clutches being used. The six-cylinder pedal can be modified but cutting, splicing, and welding is required.
As for #2
there are a couple of possibilities;
insufficient clutch departure, or a bent clutch disk, or the disc is sticking on the splines, or a faulty PP, or the centerline of the trans is not on the same centerline as is the crank.
as for #3
The interlock consists of the shift lever in the box that moves the idler, AND the reverse lever in the cover. These two are designed to make it impossible to select a forward gear with the trans still in reverse. They are a really really close-clearance fit. To make the clearance consistent, holding the cover to the trans in the exact right position is paramount, and so, holding the cover onto the box are supposed to be TWO bolts with extra long shanks, that act as dowels.
As the trans may have been taken apart many times in the past 55 years, they may not be there anymore, or they could be in another location. If the cover falls down during attachment, the clearance can become less than zero, and then you get into the situation that you describe. So then, your first step to finding a solution is to go see if those two long-shank bolts are where they belong. They should be on the vertical sides about half way between top and bottom, one on each side. Then, loosen all the bolts, jam the cover up as hard as you can, hold the cover there while tightening those two bolts. Then, engine not running, put the stick into the Reverse gate, and see how the shifter works. Move it in and out several times.
If the shifter seems to go in but then hangs up, likely the gears are butting together, due to the driveshaft not turning; just step on the clutch,which will allow the cluster to turn a hair, and try again.
The stick should snick in an out with an easy push, and a satisfying sound. If it does, yur good to go, finish tightening the bolts.
But if it still hangs up
disconnect the reverse shift rod from off the trans, and try to manually engage reverse from under the car. If it now goes, then your shifter is at fault.
But if it still won't go;
then the cover and reverse lever are NOT matched. The lever in the trans comes in a long and a short interlock section. I have never run into this, so I'm remembering this from 45 years ago when I was a manual transmission rebuilder. I forget how they were marked, so you will need a factory repair manual. What I think I remember might be that they are stamped with letters of the alphbet, Like A or B
But this I know, you cannot change that lever with the cluster still in the box .
However, you can cheat.
Find the two long-shank bolts and take them out.
loosen all the rest. Now jam that cover up as high as it will go and tighten a few bolts, then try shifting it. If the lever now goes in, tighten all the remaining bolts, Bag the long shankers, and install two standard bolts. Now yur all set until the next time the cover comes off. Save the bolts.
The proper procedure is
to change that internal lever, but the trans would have to come down, AND
IDK/you don't know, perhaps the trans already has the short interlock, so taking the trans down would be a waste of time. If this situation presents itself, you might think to machine the lever that is in the cover.
Like I said, I have never run into this situation. Nontheless, it is a possibility all these decades later.
Ok I'm all out of words
Happy HotRodding
Still had to make payments on it as no one got free cars in my family.
