Robert the high amperage is from the inrush of current when the engine is started. The coil can draw around 25 amps or so and then the amperage use declines from there so that while running it may be 8 or so and in some cases maybe less. I'm using a 30/40 amp Bosch style relay and it has worked just fine for several years. The main thing you're doing with the relay is protecting the contacts of your ignition switch. Running 20+ amps through that switch can be fatal for the contacts in there. Some of those ignition switches were only designed for 10 amps and you will find some today that are rated for 20 amps. Not common to find one (honestly) rated much higher than that...at least one that will work in the AD trucks. You will see some Chinese switches that claim to be rated for 30 amps. I don't trust that rating any further than I could throw an elephant. Too easy to use a relay and any type of switch to control it. Good luck! Please let us know what you decide.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end