hmm dunno about the mileage bein saved..if i guesse id sah no bc the cluster holds the mileage even when you swap to another truck.
the motor/tranny ALONE is 1200+ easy
pull the tranny pan an look at how dirty the fluid is. befpre going in with the engine/tranny combo change ALL fluids. also pull the valve cover itll have a discolor to the heads...light brown..55k or less, a lil darker shade 95k or less. UNLESS they didnt take care of the motor then i would be prepared for problems.
changing the cam? 220/224 112-113+4 lsa or spec a cam..your choice
putting a stall? 3000-3200 imo
shift kit is wise
3:73/4:10s depending tire height an weight
get some pics up of the engine/tranny combo
happy modding
2008 GMC CCSB 6.2/6L80e swap
Gen IV 5.3/60e swap 1994 ecsb W/T