i have the stock 2.78 open diff in my 68 cutlass and i want to add a posi with 3.23-3.73 gears if possible.

 

As I understand, you can use your 2.78 open differential (most likely with 28-spline axles) in upgrading to a 3.42 or a 3.91 posi. There are new units on the market with those gear ratios that will fit into your Type "O" differential housing and mate up with the 28-spline axles. Between the two, I'd chose the 3.42:1 posi unit - it's a good mix between performance and getting around on the freeway.

Randy C.

 

i was able to find a thread that said i needed a spacer to accept the lower gears but no information on who sells these parts.

 

I would contact monzaz on this site. I picked up a complete axle housing from him with 3.23 posi (Auburn limited slip).

He also has 3 series posi carriers, if you already have the gears and axles, to fit with your application.

For a comparison, I am pulling 3000 rpm (maybe a hair under) @ 70mph. It still cruises well (not as well as the peg leg 2.56 I had) and definitely launches better.

For my relative mild performance motor and stock torque converter/transmission, it was a nice improvement.

d1

 

If you use the correct carrier,you will not need a spacer.The posi carriers that Monzaz and myself sell,are correct for the Richmond 3:42 & 3:90 gears.They are also correct for the GM 3:08 & 3:23.