I've had 100% success on several dozen pianos, repairing separated
pinblocks, using the following procedure:

First, check to see that the plate hasn't cracked, particularly across
the gusset in the upper tenor section where the bridge is notched.  (If
it has cracked, you may or may not want to disregard all the rest...)

Lower the string tension throughout _completely_.

Remove keeper nuts from nosebolts and turn nosebolts down away from the
plate several turns.

Remove all lag screws in the upper portion of the plate.

Using C-clamps and/or bar clamps, pull the gap back together.

Visually check that the piano is square, not twisted, that surfaces
that should be parallel are, in fact, parallel.  If not, loosen _all_
the plate bolts, and throw the piano down flat on its back on the
floor. By strategically blocking up one corner, and jumping on an
adjacent corner, you should be able to straighten the case and
strongback. (This may require two people.)  At this time also verify
that other wooden structural members have not come unglued.

Drill through the original lag screw holes in preparation for
installing through-bolts.  These can be hex head machine bolts or
carriage bolts.  Recess the nuts or bolt heads as necessary on the back
side.  If there are only four or five at the top of the plate, drill
holes for several more right through the plate in areas where the
tuning pins won't interfere.

Relax the C-clamps and pour your favorite glue or epoxy into the gap in
the pinblock.  Re-tighten the C-clamps, and install the through-bolts.
Re-tighten all the remaining original screws.

Adjust the nose bolts so that they just touch the underside of the
plate, and tighten the keeper nuts.

Pull the instrument back up to pitch, distributing tension as you go,
similarly to how a cylinder head is re-torqued onto an engine block.

Re-tune several times; play the instrument every single day.

Cheers

Doug Rhodes, RPT