Its always best practice to replace the transmission cooler anytime you install a new transmission. Transmission coolers can catch and store harmful debris as well as actually inhibit cooling if its restricted.
2001-2007 GM Trucks with the 13.25" width core. LBZ trucks may be equipped with the wider core GM # 15102153. Please match to original.
Allison, 5-Speed, Automatic Transmission, External Oil Cooler, 13.25 x 5 x 1.25 in. Core