1963 SM420 transmission help?

[vinco]

I have a problem with my transmission. It will not disengage the gears even when the shifter is in neutral. Yes, I know where neutral is supposed to be and what it's supposed to feel like in the shifter, and all of that is good. The problem is that it stays engaged in gear all the time. The truck tries to lurch forward with the starter, even in neutral, unless I push in the clutch. Since there is no seat in the truck, and I usually have to work alone on it, it's darn difficult to push the clutch the whole time while trying to start it. I need neutral, and the trans can't seem to do it. Any ideas?