Well the general concensus in the old school field is 100w is ok, but its the max on standard wiring, anything higher and you should use heavier wiring and a relay. I fitted out rally cars for white road runs for years and 100watt was fine on standard.
It is "double" the loading, but the wires will be fine. If they run that close to the tolerance then the wiring would almost be a fuse in itself.
I've gone up to 100watt with my 156 and no problems at all and thats on modern wiring! Escort MK1 100W no problem, Triumph Spit 100W no problem, BMW 2002 100W no Problem.....and the list goes on. I did check the resistance of all the cars I've fitted out and all were "hot to trot".
You'll be fine my man (or possible woman in disguise), but if you step up to 120w and beyond, a relay and bigger wires will be needed to give that extra current some good ol'flow.
I did work for the cosworth rally team for a couple of seasons and believe me, when it came to wiring and night stages.....all hell broke loose.....it was a case of get me that bloody power line from the field!!!! You'll be ok..the wires have alot of tolerance, the fuse will blow if you over yank it, thats what they are there for, why else has many people already said they only relised etc etc etc....wiring and fuse fine. Its a good thing to do, BUT!!!! make sure your lights are aligned properly....there is nothing worse than a w****r with high wattage bulbs and mis-aligned light unit to ruine your day!
Good luck fella.