Mafac Racer Brake Replacement
I have a 1975 Peugeot UO18 that I turned into a commuter. I replaced the wheels with 700c contemporaries. The UO18 is the mixte version of your UO8 road bike, so almost all the components are almost the same. 

Like you, I found that the original Mafacs sucked so I replaced them with Tektro R559 Long Reach Brakes. These had the requisite 55-73mm reach that I needed to fit my smaller 700c wheels. You'll find that the main issue is that most contemporary road brakes don't have the right amount of reach so it's very slim pickings. 

A note about reach (from the comments): With my  700c wheels, I'm near the limit of 73mm required reach in the front and 70mm at the rear. With 27" you should be in the middle as the 700c is 8mm smaller than 27". You can measure your current reach quite easily with a ruler. I tossed my mafacs so can't help sorry.



I am using them with the original brake levers. I really should get contemporary levers but right now I can't ride this bike so it's in storage. 

PS: please, please do change the stem on your bike if you have a one piece aluminum one. They are known to snap with horrible consequences.