So your Grandfather's truck has the older non synchronized with reverse lockout lever on shifter; sort of left over transmission from the previous model.
The new truck has the SM420, which is very common in AD trucks, could be a little later 47 model or could have been swapped in at any point in the past 65 or so years.
To add the transmission mounted parking brake to an SM420 that didn't have one your best bet is to find a transmission that does have it and swap parts or whole transmission. I believe the tail housing is different as well as you'd need a lot of the miscellaneous pieces and parts. A good used SM420 with brake should cost less than $150, likely around $75 to free if in the right place right time.
If you do keep the wheel mounted parking brakes my understanding is the folks at Inline Tube have the cables already or can custom make whatever you'd need. I see they list 3/4 ton and I think they'd be real close if not perfect, or may need them to adjust overall length.
They also offer them in Stainless, and not much more expensive either.
http://inlinetube.com/Brake%20Cables/brake%20cable%20listings/bcables7.htmGrigg
