I had one on a Farmall Super C and couldn't tell any difference in performance. It lasted about a year and a half and failed. I put a new cheap stock type Napa coil back on. Been fine ever since.
I seem to remember that a given engine only needs "X" amount of voltage to start the air/fuel mixture burning and that anything beyond that is wasted.
I think that stock for your 420 will be fine.
Unless of course you have a blower,turbo, and are dumping huge quantities of fuel in it:)