I have only worked on a later model, but the boiler fixings seem to be the same on both according to George Schreyer's excellent site.
It's well worth exploring but your question is probably best answered by the second paragraph in the last section of the page in the link below.
http://www.girr.org/girr/tips/tips2/pacific_tips.html < Link To http://www.girr.org/girr/...ips2/pacific_tips.html
The small screw on the boiler top centre line just to the rear of the smokebox is so small its easily missed.
The locations of the other screws are clearly shown in the downloadable instruction book for the later Pacific and Mikado.
Also be prepared for the cab windows to fall out when the cab is lifted, at least they do on the later version.