The starter motor draws a tremendous amount of current, more than all the other electricals combined. For that reason it needs an excellent ground connection to work as designed. On my '48 there is a braided metal strap that is bolted to the frame and to the bottom mounting bolt of the starter motor.
In the factory assembly manual there's an entire page dedicated to grounds. Factory builds had a braided strap that was bolted to the frame and to the bellhousing on the passenger side. I'd be very surprised if yours didn't have one from the factory. If it doesn't have one now, go to the parts store (after measuring how long it needs to be) and get one and install it.
It doesn't matter where you put it so long as it connects the frame to the engine (because the battery's ground is connected to the frame.) It needs to be a braided cable because, as your throttle cable and choke cable have pointed out, a lot of current will be going through that cable. The average battery has around 400 to 500 cranking amps. (I use an 800 CCA Optima battery in my truck.) The biggest fuse a vehicle will usually have is 30 amps. So the battery's ability to push current is more than 10 times that of the biggest circuit in your truck. Wires can't handle that much current. That's why you use a braided cable. Connecting it directly to the starter is simply convenient. There's a bolt right there. It grounds the engine as well, and it's close to the battery.
