Please don't drill stems. There are 22.2mm cable stops that clamp around the stem and can be made to look as sexy as you want by polishing them
Please also get rid of that chainset, it gives me ptsd from all those Raleigh Elans that used them, they are very flexy and heavy, that nice frame deserves better
As for the braking, don't overlook 80's shimano single pivots - if they can fit, which I doubt - but they work really well
But when you do change the brakes, even the most basic period aero shimano, diacompe or anything non Weinmann lever, will be better and more comfortable on the hands
I have a similar age Elswick with Japanese Tange no. 5 cromo, Alhonga dual pivots and Shimano 500ex levers - perfectly decent braking with debadged Alex Rims wheels pretending to be older
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