Ted,
Sound like you have the right plan. You wount need a lot of cfm's for this size arch I would maybe look for a 75 cfem blower. Motor size would be a fractional HP like a 1/4 hp or maybe smaller. Check Grainger on line or maybe craigslist ?
Make sure if using a electric cord it is on a GFS breaker and that there is not as tripping hazard around the arch. About any thing that will move air will allow you to make more heat. Doesn't mean you will increase your boil rate, that would need to be checked as a before and after test. Also you will have a hotter fire and burn a little more wood. You can adjust the air flow on the output end of the blower with a adjustable gate/damper/restrictor which can be simple to elaborate.
Ahhhh yes more fire!!
Regards,
Chris