Wanted: long drop front brake (allen key)

So I've just realised that the bargain track frame I picked up a while ago is for 27" wheels and none of the spare brakes I have will reach the rim of my 700c wheels. My aim was to build it up as cheaply as possible from parts I already had, so this is a bit of a pain. Anyone got a long drop front brake they want to get rid of cheaply? It needs to be for an allen key fitting. Shimano R650 would be ideal.

Have you actually measured the drop required, cheapest way is to file out the slot on a pair of std reach to lower the pads.
I don't know how much braking power you need but I have an old single pivot Shimano SLR caliper 47-57. The return spring isn't the strongest but it's functional and has a quick release lever. No AK nut and no pads but £5 will get it posted to you.

Assuming you measure from the centre of the mounting bolt, I reckon I need about 55mm. I did think about filing the brake, but the idea of anything that weakens the brake makes me slightly nervous. I'll try and do some more exact measurements in the morning - it is pretty close on the present brake, so I wouldn't have to lose that much.
Thanks for the offer - I think I'd prefer dual pivot + I've been offered one cheaply by PM, but it's on the wrong side of London so may not get round to picking it up for a while. So will have a think about filing in the meantime.

I've filed out a 57mm long drop dual pivot (by about 5-6mm) for a rear 27". The bolt/washer clamping down the pad to the caliper arm doesn't need the excess material below it to function and given longer arms/slots already exist in same/similar materials any weakening would be negligible if any...IME/IMO