Putting 1.94 valves in your heads is common practice, like Ricky said, a good machine shop etc. probably have done several. While they are at it you could have them put screw in studs and guide plates. Also you must use the correct valve springs for them because the keeper groove is in a different location than the the 351's. PAW catalog has a set listed for this application. Did this same conversion several years back on 302 heads, still running them today on a race car. The only drawback was that when all was said and done, I had spent more money on those heads than it would have cost to buy a set of GT-40's or other after market heads, second hand etc.