When you look down the hole after removeing the gearshift lever the three rails should be lined up. The reverse has a noggin in the center of the slot that has spring pressure behind it. Put ia big screwdriver down the hole & try to line them up. When you had the cover off you probably didn,t get the reverse fork in the groove in the sliding gear. It is a little tricky. Pull the cover & put the reverse shift rail half way in reverse. Move the reverse gear so you can see the groove in the area between the other gears. Put a little pressure on the drive shaft so the gear will not slide away. Then put the cover on. Put in a few bolts & try shifting to make shure it,s right before buttoning it up.
Desregared info about the half way into reverse. That pertains to the 68 up 4 speeds. This one the top should go on in neutral.
I may have to reserve my rocking chair in the nursing home.
