I am not clear on what you are asking on the ignition.
Are you talking about the starter circuit or ignition.
Niether draws alot.
If using an after market box, MSD,Crane, etc,they have a relay built into them so none is needed.
My MSD only draws 7 amps.
If using a HEI,a relay right near it on the firewall is not a bad idea.A 30 amper should be fine.
In either case a good toggle is all that is needed.
If you are talking about the starter circuit,kinda the same deal.
The solenoid on the starter does the same thing, it is kinda like a relay in it's function.
A HD momentary push button switch is all that is needed,no relay required.
Is that what you are looking for?