I know the thread is a few days old now, but I have successfully fitted Allen Key brakes to my old 531 commuting frame. Opened out the hole in the rear of the fork crown to 8mm by drilling and fitted a thin curved washer also opened out to 8mm.
The seatstay bridge is a bit harder (difficult to get a drill in behind the seat tube), but I did manage it - again, only opened out the front hole and left the back one to ensure good location of the brake. I used a thicker curved washer, also drilled out. They are very solidly fixed, the seatstay bridge may be weakened a bit so beware if it is very slim. Different lengths of AK nut should be available from your local bike shop if you need a longer or shorter one (longer ones can always be cut down with a hacksaw).
The other way I have done it was to use the back brake on the front with a long AK nut, and with the longer stud on the front brake, fit this right through the chainstay bridge and use a normal nut/curved washer etc. This avoids drilling out the hole and weakening the seatstay bridge. Make sure the brake blocks are fitted the right way round if you swap front and back brakes.
I have also done it the other way around (old brakes onto allen key frame) by cutting short and re-threading the mounting studs and buying some allen key nuts separately.
If you have mudguards, the hole/slot in the mounting bracket will also need drilling/filing out
It's not too difficult if you have drill, vice, hacksaw, set of metric dies, file etc. but if you are not sure then don't take the risk - brakes must be mounted securely[:0]
