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Cheapest solution is to simply carry on with what you're doing, having said that, you won't regret it since it'll be very comfortable, less likely to cause a lots of vibration on the front rack, and easy to just go on a straight line and not worry about potholes.
The 650b conversion is reversible anyway, so it can be changed in future if you want to extend the bike's usability further.
Your problem is not the drop of your current calipers, but the width to allow a normal sized mudguard to fit (we're talking 45mm mudguard).
If you want it to match, you can always get the Mafac Racer centerpull, they were common in race bicycles at the time due to the lacklustre straight of the side pull brakes.
like this 650b conversion.
Personally, it's one of those deal where decent performance outweight the look of a full gruppo.