Tapered seat spark plugs are your only choice, correct? work
Are you wanting to know something else like spark plug heat range?
if so I would start with the spark plugs that the motor came with and adjust the jetting and timing to make the motor perform the best with those plugs as long as the motor is still stock wrench scopetwocents

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)