Why wont my tachometer operate properly?

1992 GMC K1500 4.3 V-6, 5 speed manual. Rebuilt: a 5.0 with all 1992 electronics.
Another concern is while driving down the road with the cruise set at 60 MPH in 5th gear the shift light comes on then goes out. The tachometer in the console reads approximately 2750, my ALDL reads approximately 1800. Also when accelerating the difference between tachometer in the console and my ALDL greatly increases. Any assistance would be greatly appreciated.
Below is what was done to the motor:

Vehicle prior to rebuild:
1992 GMC K1500 4.3 V-6; 5 Speed Manual
VIN: 2GTEK19Z3N1528893
Computer: 16171661 AZUS *86AZUSKA20200291*
Top of PROM: AZUZ 1712

Vehicle after rebuild:
1992 GMC K1500 5.0 V-8; 5 Speed Manual
Computer: 16146299
PROM Top: AWLB 7706 / Bottom: EB4(F) ZI01AZ2ME AAO3

Items completed on motor:
Bored 0.40 over; new pistons with moly rings.
Refurbished Crank; new main & connecting rod bearings.
New cam & bearings (See Below)
Stock Distributor & cap
Stock Injectors.
Exhaust System: 2.5” Dynomax; no catalytic converter
Stock Heads: new springs added as per cam.
Below are the cam specs for cam that was installed: COMP Cams Part # 12-388-4
Intake Exhaust
Valve Adjustment: HYD HYD
Gross Valve Lift: .425 .440
Duration @.006 Tappet Lift 252 260

Valve Timing Open Close
@.006 INT: 18 BTDC BTDC 54 ABDC
EXH: 66 BBDC 14 ATDC
These Specs are for cam installed @ 108.0 Intake Center Line
Intake Exhaust
Duration @.050 206 212
Lobe Lift .2830 .2930
Lobe Separation 112.0
Valve springs where installed as per directions.