Wow, thanks for all the replies.
42 tooth chainring is, indeed, the smallest I can go up front with current cranks. LBS said the rear parts were cheaper to replace than front parts. Could I replace crank without replacing bottom bracket? I don't know. He didn't seem to think so.

Also, not sure whether people just mix&match these words, but note: it's an old-fashioned freewheel, not a cassette & freehub.

I bought through the IRD website, I'll try to just get a refund rather than exchange.