I have a 1995 gmc sierra, it has a 5.7 motor. What
is the biggest transmission i can put in my truck,
The transmission your truck has can handle any load you can pull. I would install a heavy duty fan operated fluid cooler, a deep transmission pan and maybe a remote filter. You can also have your transmission beefed up to handle a lot of torque.
This might be what you want --- http://www.txchange.com/4L80Etm.htm
Looking for a Used Sierra 1500 in your area?
CarGurus has 58,860 nationwide Sierra 1500 listings starting at $1,980.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts