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It is very hard to fit C4 seat into C3. Even without rails your might have knee pushing the steering wheel. C4 seats are in front about twice that high as C3 original seats...
There is couple of guys in this forum who have done this conversion, I suppose they will chime in by themselves...
But I've heard and read that electric seats are a bit too high. Infact I sat in one C3 with electric C4 C4 seats and I had some problems with my head banging on the T-top cushions. Manual C4 seat should not be so high and when usin C3 rails it should be OK.
Manual C4 seats will fit using C3 rails. You will need a tilt coulmn to get in and out easily. They do look nice. I have a friend who put some in a 69 vert.