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Ok. Good questions.
Porsche had 4 strokes in air cooled engines: 66mm, 70.4mm, 74.4mm and 76.4.
66mm was '65-71, 2.0/2.2 (changed bore)
70.4mm was '72-83, 2.4/2.7/2.8/3.0 (changed bores). The crank was changed from 6-bolt to 9-bolt flywheel end in '78 w/ the SC motor.
74.4-3.2L Carrera, 3.3 Turbo (changed bores)
76.4-3.6L 964/993
The large base 4B cams came out in the '78 engine as well, along with the stronger late aluminum case.
I dont know what youre doing with the car, but I was proposing essentially a 3.0RSR engine if you were looking to do something like a radical 2.8. 95x70.4 (standard stuff), hot cams, high compression, ports, good hardware (head studs and rod bolts). Essentially start with an SC engine. There are lots more details but this should keep you going !
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