jj44;
You need to provide more information on the blind plate for hydrostatic testing. Is this a through-hole (through thickness) or a partially drilled hole in a blind plate? You may need to use a dished head for testing.
If this is a test blind then the hole is through the blind, and this is a 36"-600#RF blind? ...if so the thickness is 6.625". Also you should NOT use NPT threaded piping in that pressure range! Leak/failure with threaded piping is too great, I'd suggest Socket-Welding the nipple in place, plus bump up the size to 1" min.