The 15 amps is probably the draw. (what it takes to energize the solenoid) As you said the delco solenoid on your starter can draw 30 amps or even more. If you are only going to use a relay to energize the delco solenoid at the S terminal the 60 amp bosch would be fine. If you want to use the ford solenoid to do the same thing it would also be fine, but a bit of over kill. If you want to connect the S terminal to the positive terminal at the starter solenoid, you will need to use the ford solenoid. This will make the heavy wire to the delco solenoid from the ford solenoid dead ( no voltage) until the ford solenoid is energized through the ign switch. The ford solenoid can carry well over 500 amps to feed the starter and solenoid.
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