I don't know if this has been answered yet, but I have a new 4T65E 2000+ refurbished Transmission on a crate and it will undoubtedly fit in my mom'* 2003 Buick Century (M15). I also have the same code and trans type, but will the transmission bolt up to my car, 1999 Pontiac Bonneville?
A mechanic said that bolt pattern doesn't line up but I also read that the torque convertor bolts don't line up, and the mechanic didn't mention exactly what. so what makes it incompatible? I am wondering what will make that 4T65E transmission for 00+ vehicles bolt into my car which is like 97, 98,99 vehicles, nonsupercharged of course.
Thanks for your input, will save me lots in getting a new tranny

I don't know where the mechanic is getting his info from, but if its a 3800 Series 2. The bolt pattern is the same from 1995-2005. And usually, the flexplate is different between the L67 and L36. Just keep both of your parts and match them up.

this transmission is for NA motors, so it is not the HD version. I am still wondering if the transmissions from 2000+ vehicles will fit my 1999 Bonneville, or any pre-2000. like i said, the mechanic said it doesn't line up so I had to order one that does. what is the difference between the years of these transmissions and is there a way or adapter for them to work? Please help if or why these are or aren't compatible and also, does the body type matter? like between H and W and whatever a monte carlo is and buick century. also, same similar engines could they fit in older vehicles?

The transmission will bolt up fine but the two things that need to be checked for compatiblaty are if the replacement transmission has a internal neutral safety switch and if the replacement transmission has the same final drive. The 4t65e had four different final driver ratios.

This link will help decipher what year and what ratio your transmissions are.
GM FWD Transmission ID charts