What battery replacement works with a 90 348ts?

Yes, Optimas are compatible.
I run an Odyssey 680 in mine. Saves 25 lbs., but it is only used for starting the car. No stereo. If you run a big thumpa-thumpa stereo in the parking lot of the car show, you'll do best with a lead acid battery, as the Optima in my experience doesn't have the staying power of a lead acid. The Optima won't last as long as a standard lead acid, but looks cooler and won't spill acid if you roll the car. You can mount it sidways or upside down too.

 

Just for the record, having the parking lights for few hours drained a typical battery while optima battery was able to crack the engine after 8hours.

 

I've used this battery for over a year without issues. http://www.braillebattery.com/index.php/braille/product_batteries/b3121

Plenty of juice to turn the engine over even in a cold garage. It saved 24 lbs over the battery that it replaced.

 

Sweet. When my 3 year old Odyssey dies, I'm gettin' one of these! Battery tech has come a long way!

 

Running an Odyssey pc925 in my car at the moment.