They probably used 1600v caps because those are AC line filters. Justradios.com has an excellent article on replacing such caps:
http://www.justradios.com/safetytips.html
I believe the ideal would be to replace them with type Y "line to ground" caps with the original voltage rating or higher.
Next best in my book would be to leave in the 1600V caps if they're something newish like orange drops. If they're plastic-coated paper (bumblebees, black beauties, etc.) then out they go -- as unreliable as paper.
Last choice would be to replace 'em with conventional 600V caps. If you're going to install new caps, why not spend a few more cents and use the cap designed for that application?
Just my $0.02.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html