If the bridge just has a single diode in each leg than the forward drop should be similar to a lower voltage bridge.
If it has more than one diode, then obviously the forward drop (and losses) would be higher.
You can measure the forward drop to determine which it is. A single diode will have a forward drop of about 0.7V at a few mA of current.
Apart from the possible high forward drop, rectifier diodes with that much junction area may have a high reverse leakage current. This would give you some odd results if your low voltage power supply were not loaded. You can check how the diodes perform put say an amp through them and see what the forward drp is. It should be around 600mV. Then put about 12V reverse voltage on the diode and measure the leakage current.
But my feeling is that the rectifier diodes will be OK for your lower voltage power supply.
50 amp, 1600 volt, wow!
Are those rectifiers brand name, like Semikron?
If so, they are quite expensive. Sell them on ebay. And purchase yourself a good bin of small signal electronic components to stock your lab.
They are only expensive if you don't work with that sort of stuff and know where to shop.
I say keep em and put them to good use in lower voltage projects. In a 12 - 24 volt application 1600 volt 50 amp rectifier is going to be really hard to kill. Gross overheating due to inadequate heatsinking would be about it.
Just don't be surprised if you see a .9 - 1+ volt forward drop if you are running them near a 50 amp RMS current level. The HV rated diodes tend to do that.