Firstly I have never done a splitty roof but I would imagine the general process is the same as others and I have done a few others.
1. Make sure you are swapping apples with apples, check all measurements more than twice.
2. Dependant upon the condition of the donor and damaged vehicle pick a consistent line to cut through so you can do neat strong welds when you are putting it back on. A bit of planning now will save you a lot of heartache later.
3. Make sure you use plenty of temporary bracing in the body so things dont warp when you take the roof off.
4. Make sure you take plenty of photos that you can put up on the forum for the next bloke who asks,
Am sure someone can offer some more specific details, but good luck.
Peter
VW Toys
64 Beetle - club car - Herbie
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samba has a lot of pics of roof swaps and repairs on splittys
remember on curved surfaces - string is your friend
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