Ian,
I am in the process of doing just that although my car is a Type 45 that someone had put a roof on!!
Before you think about cutting the roof you need to obtain the top part of the windscreen frame from one of the usual sources. Then decide where to cut and join, the choice being largely up to you. I chose roughly midway up the screen pillars to enable me to clamp the stumps of the original pillars to the new top part. This was to ensure that the rake of the screen frame would be correct. It is obviously essential to get the screen aperture the right size and the old rule of measuring twice and cutting once is a good one
I added steel rod into the lower corners of the screen frame to add strength.
The rear edge of the roof at the bottom of the rear screen aperture (IE where the hood will attach) will need to be built up. You can buy this section and laminate it in. I chose to build mine up around a former but either method works.
You will need
Screen frame top section (as above)
Rear edge section (as above)
Hood Frame (Susan Miller has them)
Hood Stowage tray
Edge trim for Stowage Tray
Aluminium strip for stowage tray (Along the edge where the hood cover attaches)
Hood Cover
Hood
Tenax Fasteners
Hood grab Rail (goes on top of the screen frame)
Cotton reels for hood frame
Drip rails (on the screen pillars)
Screen tensioner rod and fittings
Sundry screws rivets etc
Probably a new back fibre board?
Hope I haven't forgot too much!
I have seen kits of parts advertised I think at C Neil's but it might be better to shop around and look for quality bits if you have the time.
I have completed my shell and trial fitted the screen and am now at the point of building the car up as time/money permits so my fingers are still crossed
)
Hope this helps
I forgot to mention the two expensive little chrome end finishers which go on the rear edge of the door aperture -- mainly because I cannot bring myself to pay what they cost!