The 4L60E is the original transmission in that car, and while it has its weaknesses (especially concerning the 2-3 clutch pack and the 1-2 shift), it's the most readily available, parts are plentiful, and many shops are familiar with rebuilding them. A properly rebuilt 4L60E with upgraded components (e.g., stronger clutches, improved valve body) will be significantly more reliable and durable than a worn-out original.
While other transmissions *might* fit (after extensive modification), they'd likely involve far more cost, complexity, and potentially compromise the car's intended performance characteristics. Sticking with a rebuilt or upgraded 4L60E is the most practical and cost-effective approach for maintaining or improving the Z28's performance and reliability. Consider a reputable transmission shop specializing in performance vehicles for the best results.
