You could get an approximation using the Bretton Woods gold relationship. Try calculating the value of Gulden in gold during your period, and multiplying it by the value of gold today. Note that this doesn't take into account inflation and other factors, as the Bretton Woods peg didn't take it into account either.
Other methods can be, as mentioned previously, using the euro - gulden ex. rate at the introduction of the gulden, or a linear regression as an approximation, ARIMA modeling, etc.
Alternatively, if you are interested only in the effect of inflation, calculate a total inflation multiplier (that is, the effect inflation has had over the years on diminishing the value of (today's imaginary) gulden) and multiply the gulden's 1950 value by it. Again though, since Bretton Woods didn't allow inflation to naturally adjust the price of the currency, your results may be incorrect. You may want to adjust it using the U.S. inflation rate over the same period, as 1950s gulden was pegged to it.
As pointed out previously, you can't really get the value today, as there have been multiple monetary changes over the years.
I hope this puts you on the right path.