hello there!, my son has a 1980 Cutlass 2door that he bought for his first car.we r doing a father,son build.It came pretty much stock but has a nice 350 chevy motor with add-ons. I just finished putting a th350 w/stge 2 shift kit.now i need to complete the drivetrain with a POSI rear end intead of the 1 tire fire!also would love to get the gearing down to around 3:42ISH instead of the stock 2:89,3:08 or w/e it is. what is the best way for me to do this?when changing rear ends to say a 8.5,10 bolt from 7.5,10 bolt, is there always gonna be some fabbing to do? ie.changing locations of spring perch,shock mounts,upper and lower control arms?is there a rear end out of something else that is real close?something that could eventually handle up to 400-500hp.i found one that was real close w/ a funky sway bar with 3:42 and posi but was 54" wide instaed of the 51" that mine is.and bottom shock brackets/cntrl arm looked a tiny bit different ..he only wanted a $100.thanks a million for any help!take care!

 

Grand National rears or later 442 rears ('86-'87?) are a nice bolt in but can be pricey. I believe GN's were 3.42 and 442's can be as low as 3.73.

 

10 bolt 7.5 can be upgraded to stronger parts
-Eaton performance posi 28 spline
-True Trac Helical gear posi unit 28 spline
-28 spline aftermarket alloy axle shafts Superior gear and axle Moser engineering
-Aftermarket Gears made of better mix of metal for better strength and heat resistance.
These are a few things to help the 7.5 live under higher horse motors.

Bottom line someone HAS to BUILD the rear correctly also.

10 bolt 8.5 1984-1987 442 and or Grand National rears are Excellent upgrades only requiring a drive shaft 3/4" -1" shortening .

10 bolt 8.5 posi and axle upgrades are also available to 30 spline and will take gobs of torque and horsepower.

I think we have a stock 3.42 posi 8.5 on the shelf still needs reconditioning.

Jim
JD

 

I had a '80 Malibu with about the same setup - took it to a Chevy dealer I used to work for and got 10% over on the parts, and got charged 1 hr. on labor.
Couldn't believe the difference - like I got a first gear!
Out the door for around $400 in 80 minutes.
3:42 posi, but gas was under $2.
Never had a problem, and had a 2400 converter.