What would cause serious hesitation after engine was worked on? on 1999 Chevrolet Express 3500

After getting help replacing a rod bearing, my van has serious hesitation problems when I try to accelerate from a stop. Checked distributor cap, wires, firing order and vacuum lines. Also my volt gage seems to read backwards. Must be some connection. Afraid I'll do damage if I drive it far enough to get a code reading.

Asked by for the 1999 Chevrolet Express 3500