455/Thm 400 was the only engine/tranny; I think dual exhaust was optional.
As far as compression it's listed around 8.2 :1 however, I have found factory ratings to be off quite a bit at times, mine is what I call "Farm tractor" compression 7.6 :1. My new 455 is right at 9.4 :1 so it should be more powerful.
As far as rear end goes - most luxurycars were tall geared Id guess between 3.23 to 2.56. After the '73 Fuel crisis cars were really geared tall for fuel economy.
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Two 1975 455 Grandvilles &
'79 455 Trans Am
‘69 Camaro SS 396/375 (owned since ‘88)
‘67 Corvette 427/435
‘23 Lexus LS500 awd
‘20 Audi R8 & ‘25 Toyota RAV4 (wife’s cars)
‘95 Ford F250 4wd 7.3 p-stroke
& countless Jeeps & off road vehicles & motorcycles 
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