I had the stock 4T65E of my 2000 Regal GSE rebuilt and seriously upgraded in 2012 but I never did post the details of that upgrade to this forum and I wanted to go back and do that. I am VERY happy with the results and it's worth sharing what was done, and that is the purpose of this post.
The work was performed by Max Speed, Inc. which is a franchise of Speedy Transmissions.
They are located at 850 Sandy Plains Road Marietta, GA 30066.
When I took the car in, the predicable missing 4th gear was the main issue.
I specifically requested that they "Beef Up" the transmission. I said I wanted them to make it "STRONG", and to apply any upgrades as they would if this were their own transmission. I made it clear that I wasn't worried about the price, I just wanted this thing "STRONG". The following is a list of what was replaced/installed, as dictated from their receipt to me:
Master Rebuild Kit
Pressure Control Solenoid
Kevlar Reverse Servo Band
Kevlar Forward Band
Kevlar 1st & 2nd Clutch
4th Gear Clutch Shaft
Sun Gear Shaft
Parts total was 2109.87
Labor total was 994.78
Total amount paid in July of 2012 was 3131.24 including tax.
I drove the car until 2014 and then stored the car for 6 years and I just put the car back on the road about 2 months ago so the transmission has only about 2 years of use on it. Indeed it IS strong now, and a worthy transaxle for my 3800. I can no longer say that my transaxle is a weak link at all. Also interestingly enough, the car does not "torque steer" as it used to before the rebuild which is a nice change. To date, I have never had any further issues whatsoever with this car's 4T65E.
Hopefully this list of upgrades will serve as a useful reference if anyone out there needs their stock 4T65E to be upgraded to something strong enough to be worthy of their 3800 L67. I couldn't be happier with what Max Speed, Inc. did to mine.