I’ve got a 1970 cutlass supreme with a 455 with 600 hp and a th400 trans I was wanting to know would I be able to run 3.42 gears and not be screaming down the highway doing 75 and what would the stock gear ratio be

 

There will be some more qualified than i on this subject ,but the short answer is no.The reason i say this is because i have a 455 w/turbo 400 and 3:23 gears and going down the highway at 63 mph im turning 3000 rpms. Also i still have the 14 inch tires ,15in tires will help .Theres a good chance the stock rear was 2:73 . There are online calculators that will give you a good idea where you'll be at. For example, if you have 27 inch tall tires you'll be at about 3000 rpms at 70mph you current gears

 

Top performance gear option for a 1970 Cutlass Supreme Holiday Coupe 455 CID, THM 400 was 3.42

At 75 mph the engine will be turning about 3500 rpm with a 26 inch tall tire.

Your engine should be fine, although its gonna sound like its working hard. Nature of the beast back then.


https://www.automobile-catalog.com/auta_details1.php

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

 

I currently have 3:42 gears with an 200-4R OD... I did the switch to OD because of the revs on the highway cruising at 70-75 mph with my old th350 tranny with 15" tires. I love the gears, sure woke the car up....

 

2.56 was stock base standard gear on 1970 Cutlass Supreme H C with 455

G78-14 was standard tire for this model.

All possible ratios were as follows 2.56 2.78 3.08 3.23 3.42

PS so cool that 455 was a factory option for your car, not the case for 1969.

 

342 is a nice all around ratio
Put on some short / race tires
Go to the track and have FUN