Multiple problems

I have a 98 Silverado. We have rebuild the 5.7 vortec motor and the 4L60E transmission. Everything is stock in the motor except we went with flat top pistons and we had it bored out 30 over and has a new fuel injector spider. Transmission was upgraded with 3-4 clutch pack. Has rebuilt torque converter and new park/neutral switch.

TRANSMISSION: From the day we put the transmission in we have had problems. SES light came on instantly. Going into default gear but not all the time. Turn truck off and it is fine. Throwing the PO785 code. We have checked the ignition switch and have changed both the 1-2 solenoids and one of the 3-2 downshift solenoids. Waiting for the other to come in.

DASH LIGHTS: When I turn my headlights on my dash lights blow a fuse. Radio works fine as do my turn signals. Have replaced fuses and the headlight switch. Still doing it.

MOTOR: The motor has a loopy idle mostly while truck is in gear. Truck also surges under a load like climbing hills, etc. At start up sometimes dies or the RPMs drop then pick right back up. Also gets horrible gas mileage, like 12 miles to gallon compared to 18 before we rebuilt the motor.

Any answers to any or all of our problems would be great. Have spent a great deal of money on this truck to get it right and getting very frustrated that things are not working out.