The floor will flex a bit with more passengers in the car, and flex more over bumps and under load. I agree that no way it should move 1”, but what I was trying to say is leaving extra clearance for the driveshaft is much better than not leaving enough… replacing the tunnel is not a job you want to do twice.
It’s also a good idea to leave some extra room just in case you ever change the transmission or rear end later and things have to be adjusted a bit for proper u-joint working angles (different transmission length, different rear end style with a different pinion height relative to the axle centerline, etc.)
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1969 Chevelle
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: LS2, 3.0 Whipple, T56 Magnum, torque arm & watts link, Wilwood Aero6/4 brakes, Mk60 ABS, Vaporworx, floater 9" rear, etc.
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