"Still, 3'' extra just for tacking?"
 
Yes. Get the right one and don't worry about pretesting or prefitting, just put it on. Mine (3-window) is as tight as a drum. 
 
Suggestion: be very arduous/careful at the front corners, both top and bottom. You will love the fit and look if you do that. Any "adjustments" can be made at the rear where it's hidden, but I doubt you will need to make any.
 
These things are very well planned and made. Exceptional for one to be made incorrectly and needing the installer to make any kind of major "accommodation" to make them fit correctly.
 
That's my $0.02 .... YMMV
 
Bob Stevens
'87 Syncro Westy
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