Gear ratio for rear with GM Vortec Engine

I'm looking at 97 Thunderbird or Mark VIII irs for my '37 Buick.
1) Can anyone recommend the Best 97 Thunderbird rear gear ratio with GM Vortec LH6 or LC9 and a 5 or 6 speed automatic transmission? I want the car to be a daily driver so probably the best gas mileage that still feels like the car is not underpowered. The combination will be in a '37 Buick Roadmaster with an overall weight of 3700 to 3800 lbs.
2) Is positraction important?
3) Is alum of the Mark VIII worthwhile? How much does it save in weight?
4) Can I tell what a rear en is by looking at it?
Thanks

they will have a taqg with the ratio and if a limited slip

Like Jerry says the OEM rear should have a tag or you can turn the axle one turn and count the pinion yoke turns to get close, then compare to the OEM ratios from that year to dial in what you have. For your ratio you need to plug in your rear tire diameter and final tranny drive ratio, then pick your optimum cruise speed. Here's a link to an easy calculator that you can use RICHMOND Street Performance Calculator On mine I run 1995 at 70mph, which works pretty good for me but you need to look at your power band (torque curve) on the engine, and what your speed range will be. Some of the newer OEM's are only turning around 1600 at highway speeds to push economy, but you have to stay in your power band or it will be going in & out of OD all the time. I like positraction, but for your Buick cruiser I would not think you really need it unless you're planning for winter driving? Not sure of the weight, but I'd say you'll shave off 150 to 200#, and with that heavy Buick that helps.

Aluminum is 1/3 the weight of iron, however not all parts will be aluminum---you might end up saving 20 to 40 lbs

Thanks for the feedback. Found '97 Thunderbird rear assy complete with rotors and brakes. It's not positraction, but only has 16000 miles!! Paid $225. Rotors are right at nominal for new rotors. Should work fine.
1) Does anyone know if the rust on the rotors should be cleaned off by machining, or is there a different method?
2) This rear axle has sat outside for quite a few years so the boots will need to be replaced. Is it likely the rear end seals and brake caliper will also need replacement?
Thanks

1) I would take a can of WD40 and some #000 or #0000 steel wool and work the rotors over and see if they're smooth or pitted. If pitted too badly then carry them in to your local parts house and have them take a clean up pass off of them to make them new & true.
2) It's probably good insurance (peace of mind) to rebuild the calipers, and ensure that they're not rusted up from any moisture that might have been in the fluid over the years. Same for the seals, IMO.