Only if it is a 1200 amp frame with a trip point of 1128 or lower.
You are likely looking at 240.4, Protection of conductors.
For this application I'm usuing 450.3B, note 1. So 1200A is satispactory for the xfm secondary protection. The conductor ampacity has to be 1200A or more. 4-350kcmil is a nice fit
The tone of the OP led me to believe the 1200A and 800A CBs were molded case - which, of course might not be.
As for a 1200A secondary main on a 1000kva xfm - that would be the first one I ever saw that was designed for a 1000kva xfm and not just thrown together with what they had on hand. I'm sure that someone somewhere has done that, but I wouldn't know why.
As for using a 1600A set down to 1500, 4-500kcmil is a nice fit. A 1600A secondary main doesn't fit quite so nicely.
I don't think I'm goofed up on this - but I could be
In any case, the best post so far is from bobsherwood. Go measure it and match the pictures - everything else is guess work
cf