Based on my research, I could go from current 3.05 (non-HD) gears to 3.29 (non-HD) in the '98 LeSabre by swapping in a differential and sun gear from a 3.29 W-Body (or 2.86 H-Body) and a 2.86 H-Body half axle.
(This works because the drive/driven sprockets on my transaxle are 35/35 (like a Grand Prix GT), rather than 33/37 like a 2.86 Bonneville.)

If I go with used parts from reputable supplier(*), it looks like I could do this for around $200.

I can reprogram the PCM'* tranny parameters with the PowrTuner.

So here'* where I need your opinions:
From a performance standpoint, is this swap (especially the labor :P ) worthwhile?

depend on what youre looking for .to get better time at the drag its a yes (if you don't spin ) because at the 1/4 miles drag im still using only my 1st and second gears whit 3.05 .but there a price at cruising speed you will see maybe 200 rpm more and that will cost you maybe 1 or 2 mpg

Billboost went fron the stock 2.93 to the 3.29, he sould be the one to know

Gear ratio changes in a L36 would have a greater effect then it would in the L67.
The L67 has a much more consistant powerband due to it'* flatter torque curve.

In the L36 it'* all about geting to your powerband quicker since the torque curve is not as consistant.

So, I think it would be worth it.

I enjoyed having 3.73 gears from the standpoint of quickness, but the fuel mileage wasn't great (15-17mpg in town). The 3.43 ratio I currently have is still fun, but helps the mileage a little more. I would recommend it based on my experiences.