If you are rebuilding an HO and are being so specific with your restoration, the change to the original 3.36 gearing is a no brainer.
Most HO's came with 3.55 gearing for manuals and 3.36 for autos.
Will it affect? You will NOT be happy with 2.73 gearing in your HO. 2.73 gears are a dog.
Had a 68 400 manual (numbers matching, engine rebuilt to spec) with 3.36 gearing and was awesome. If you plan on a lower compression engine with a hp decrease, go with a 3.08....but to each his own.
Si Vis Pacem Parabellum
1967 Starlight black PMD Engineering 400 Auto 1968 Alpine Blue 400 4 speed 1968 Verdoro Green 400 HO 4 speed 2013 1LE 2SS/RS Inferno Orange Camaro.