I 2nd 6sally6.  If not for more performance why do you need a limited slip?  Limited slips are for more traction, more traction is for better performance.  The differential has nothing to do with how the car cruises. 

I have an AOD and a rear end I just put 3.55's into.  The previous rear had 3.20 and ran fine city and highway.  Newly rebuilt rear had 2.79 and that was not ideal  Not quite going down the road yet but I'll ley you all know.
Find a ratio and RPM calculator online  Punch in some numbers and It will give you some fairly accurate RPMs at different speeds. 

Okay...
I own and reguarly drive one ride wtih a Detroit Locker, one with a torque biasing differential, and one open differential.

Each have their purpose.

Options...
1) Locker.  Harsh engagement, absolute 100% power available to both drive wheels when one starts to slip.  Can be a bit sketchy on the street (kind of an understatement), especially in a manual transmission car where it gets “unloaded” during a gear change.  Normally only used on a car that will see a lot of time on the strip or a serious 4x4.

2)Limited Slip.  Has friction clutches inside that connect the two axle shafts.  Can have varying levels of friction “grab” depending on model and how it was set up. Requires use of friction modifier in gear oil.  They do wear and do not last forever.  When they slip they make heat.  Can be very street friendly.  Drives normally with no bad manners unless it is an “extreme” setup with very heavy clutch pressure.  Good all around choice.

Torque biasing.  Detroit Tru-trac, Quaife and a few others make this design.  Not going to pretend to know how it works.  Insides look like a magic pencil sharpener, lots of helical gears.  No friction material to wear out.  Very street friendly.  Strong, run conventional EP oil.  My go to choice for a car.

Open.  One legged... single black streak.  Lame...

Gears...
If you are installing an OD transmission with stock 2.75-3.00:1 gears you must be trying to do one of two things...
1) Set land speed records.
2) Attempt Prius like fuel economy.

I would consider changing gears to improve performance.  Use one of the online calculators to figure out engine RPM with your tire OD and potential gear choices to see what you prefer.

FWIW I have 3.5:1 gears and a Detroit Tru-trac torque biasing in my mustang.  Can’t comment on what it is like since my car is in about 1000 pieces.

I do have a daily driver with a torque biasing differential.  Still works great after nearly 200,000 miles and punishment behind a diesel.

Hi, if you want a limited slip diff for  your relaxing cruising on 8 inch you have some choice as stated from bentworker . Eaton ,for example could be a top notch choice , and you will dream well for many years.
If you have read my post about Power grip pro you have found enough info to think about .
Now , using the car , i can say is working really good .
If you want to drop rev stay 3,40. \ 3,50
Mine 3,89 is little too short  for highway  speed at 80 mph

gkm24 wrote:

Also fitting an AOD to the car and wondered whether I would need to change the ratio on the rear end.
I don't intend to race or track the car but just want it to cruise nicely down the freeway.

When I put a T-5 overdrive transmission in my car I didn't change gears right away.  I drove for about 4 years with the T-5 and 2.79:1 rear gears.  It worked fine... I just couldn't really use overdrive except for at highway speeds.
Having said that, I changed to 3.25 gears last year and I enjoy it more.

So, you don't necessarily "need" to change to lower gears when you install the AOD... but you'll probably enjoy it more once you do.
 

The other issue with any torque sensing differential is that they take a small amount of time to react.  A clutch type limited slip works immediately so long as the breakaway torque is not exceeded.  A locker works the same way, but with a more aggressive and stronger lock.  Selectable lockers are literally light switch.  They use either compressed air or electromagnets to go from open to locked at the flip of a switch.  There are selectable lockers in the off road world for some applications that function using a cable as well. 

IMO, on the street a good limited slip will last a fairly long time in a toy car that doesn't see DD mileage.  The stock Ford Traction Loks aren't great.  Typically they are a weak 3 pinion design and are short on clutch surface area.  Eaton builds a nice limited slip, as does Yukon.  I tend to run Yukon stuff, and have their limited slips in three different vehicles. 

Lockers are more for a street/strip car that legitimately spends a lot of time at the strip.  In a light car they are particularly harsh in operation.  A selectable locker gives you the best of both worlds, but there's added complexity, and they are expensive.

In a real race car you just run a spool.  I mean if you're turning going down the strip you've got problems a lot bigger than your differential...

But, the diatribe on differentials aside, unless there's a need for additional traction changing the differential for the OP's intended use is just a waste of money.  He talks about maybe needing to change the ratio, but never mentions what the ratio is that's currently in the car.  At this point I would say with any overdrive trans you are better off with a gear ratio in the 3.20-3.80 range depending on the exact model trans, tire size, and intended use.  If the car had say a 3.00 would it be worth the expense to swap it to a 3.25?  Probably not IMO, and if you're not already in the rear for a gear swap there's really no reason to touch the differential.