Hi

My A8 4.2 1997 transmission is dying and i found one that came off a 1998 A8 4.2L. Will it fit with the 1997 TCM or will it fit with the 1998 tcm? thanks

The 97 TCM should work with a 98 trans.
The TCM values reset after 15 minutes disconnected from battery, should re-adapt to new trans shifting. It would do this with the same trans, anyway.

And physically/mechanically will it bolt right on ?

It should...nothing really changed externally 97-98.

Myself, I would be finding the 3 letter trans code of my present and replacement transmissions and discussing it with the qualified mechanic that's gonna do the swap...it ain't no home driveway job. The code is on the metal ID plate on the bottom of the trans and on the vehicle ID label in your spare tire well.