To get to the original question, The S terminal on the ignition switch can be ignored. The wires for the starter are as follows. R, is not needed. S is to the push button and the remaining is the Battery as you mentioned. The other post on the push button is to the ignition circuit.Use the same wire that goes to the coil. If you want to keep the "stomp" starter for the truck, then match the starter to the flywheel. A 6 volt starter to a 6 volt flywheel. If you have the original flywheel on your 61 261 then use a 12 volt starter from a 55 to 57 Task Force truck and keep the "stomp" stuff on the bell housing. You will need the bell housing from the 216 to fit the 261 into your truck. If you need to use the bell housing from the 261 the mounts are angled and you will need to adjust the cross member for that.

Steve H