I would go in a couple of passes if I had the reamers already on hand, but at 60-100 bucks i'd at least try the larger one first if you can get a real long tapered one.
That cast iron cut a lot easier than I thought it would, just tale it slow. I was lucky and had found a used 4 flute multi-step reamer. I should have kept it, it was great... NEW they are $$$$
It was a lot easier with the stepped reamer than when I did a friends with the tapered one, but even that was not really that hard. I did not use the chinese cheapey reamers though, no idea on the steel quality there.