I've put them in a 993, not a big deal. Almost bolt-in. I've seen 993 seats in an 80's Carrera so I'm sure your mod can be done without major drama.
Be forewarned they sit a lot higher than previous generation seats. I did not like them in the 993 for that reason and I'm barely 5'9".
I'm pretty sure the the main mod is bending the front mounting tab on the seat rails "flat" from its 90 degree inclination. Up though the 993 series, all 911 seat bolt holes go vertically straight down into the floor. Starting with the 986/996, the front mounting holes are closer to horizontal.
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Kie in Seattle. '05 3.4 987, '76 930 DP/Kremer thing. Past p's... '91 928S4, '95 993, '82 928, '91 944S2, '92 964, '67 912/911 2.7, '72 911/3.0, '89 951, '87 951, '94 968 cab, '96 993, '87 928S4, '78 911SC targa, '79 911SC, '84 Carrera row, '75 911S, '85 Carrera targa, '84 930 RUF, '04 996 C4S.
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