I looked in the manual and couldn't find anything.. (doesn't mean it's not there...)
The hand brake goes under the tranny cover, but it seems like the speedo went through the firewall, and with the steering, brake and clutch pedals and shift linkage all in the same area, I'm not sure how the cable runs....
It's different between 48-50 and 51-52, not sure about 53-56 and beyond. On 51-52 it goes thru a hole in the firewall right near center of the firewall, loops down and goes along the trans to the rear. It shares the hole in the firewall with the choke cable and throttle cable. Just keep it away from electrical stuff in the engine compartment (my cable has a bare steel sheath, some are plastic coated).
On my 53 [not restored] I ran the emergency brake cable and the speedo cable both thru the recess in the upper drivers side of trans cover. I was sure that the factory manual showed this, but a quick look thru did not find it. I may be wrong about that being "factory".
My 53 is no help because it is darksided with electric speedo, but the 53 parts truck still has the cable in place. As far as I can tell, it comes thru the firewall several inches above the steering column, between the column and inner fender, between the brake and clutch pedals and from there I'm not sure because it seems to loop under the trans. This could be related to the PO installed 289 and 4 speed