A shift kit on an 80 with good clutches will be fine. Your t-case and front diff will be next to go. Speed engineering is good for the harness, but for a crossmember, go get one from a 2500hd 4wd. Mine bolted right up to the frame with just 4 new holes.
2003 GMC RC SB LQ4 4L80E RADIX 2.8 pulley 8psi 472RWHP/495RWTQ (w/OEM throttle body) 91 octane no NOS no CH3OH lingenfelter GT-7 cam 1.65/2.02 rev valves w/comp 918 springs and pushrods one off 9.2 CP pistons and manley rods JBA titanium shorties magnaflow 3" catback 90mm LS2 throttle body conversion volant CAI w/ram air 4.10 eaton posi SSBC 14" front brakes w/4 pot calipers one off intro 20x8.5 20x10 "bean 6 covered" w/graphite powdercoat covered in 265/50 and 295/45 KDW2 rubber hotchkis front and rear bars beltech 4" rear w/c notch and caltracs eibach 2" fronts edelbrock AIS shocks suede/leather/ostrich denali interior w/bucket seats painted power fold signal mirrors all chrome powdercoated black and shaved tailgate black speed grilles snugtop UPCOMING: ASM LTs w/ 2.5" dual magnaflow, new tune (w/dyno), slicks...