2005 Silverado 4.3 to 5.3 throttle troubles

I have a 2005 Silverado 2wd, originally a 4.3 but when that blew I swapped out for a 5.3. The motor is a 2003 I purchased complete from the junkyard - harness, tac module, gas pedal, all of it. I've got the motor in and completely hooked up, but when I try to start it I run into issues. When you try to start it sometimes it will just roll and roll without firing at all, sometimes it will fire but not start, and a couple times it has started up and idled for 20 or 30 seconds and then died out like it was starved for fuel or air. When you turn the key you can hear the throttle body click but I have absolutely no response from the gas pedal. There are no DTC's being thrown, and the VATS has been removed. I've replaced the crank, cam, and map sensors, checked my fuel pressure, cleaned the fuel injectors, and checked every cylinder for spark. There are no blown fuses, I've checked for breaks in the wiring between the throttle body and tac module and gas pedal. The ECM was reprogrammed by 150tunes. I've been to several dealerships and all they can tell me is to check my grounds, and I've been to a couple of smaller shops but they won't even look at the truck because it's "custom". Has anybody run into this issue before, or have any ideas what could be causing it? At this point I'm nearly ready to cut my losses and torch the whole truck, I've been messing with it for weeks with absolutely no luck.

It sounds like the passlock in the ignition switch is not programmed. It cuts the fuel and wont let it start. Try jumping power to the fuel pump relay. The truck should still start and run with a bad tps and/or loose grounds, it will go into reduced engine power. When you had the computer remapped did the ask for your vin. Contact Nelson Performance and they will program and mail you the right tune with a performance tune as well. Just follow the relearn procedure.

No pedal to throttle body movement says a lot.
The pedal will control the throttle body with the key on, ign off.
If that doesn't happen, check wiring and check the OS that the pcm has.
Be sure there is comm to the computer, if so, check for pedal values with a scanner.
If something is hooked up incorrectly or there is bad wiring, a code will set; unless they are turned off.

The truck was originally the base model work truck, it doesn't have the chip key passlock system according to the dealer. The ECM was reprogrammed to my VIN #. I have searched for bad wiring with no success so far. I tried replacing the throttle body and the truck will now start and idle, but I still have no response from the gas pedal. I tried the pedal from a running truck and still had no success.

if the pcm was programmed with your vin, it is not a compatible OS and parameters for a drive by wire setup.

How hard would it be to just run a cable throttle body? I have all of the parts for a DBC setup so the programming would be the only question. The truck was originally DBC and the ECM currently in it was originally a V6 ECM.

My truck originally had a 4.3 with a DBC throttle body. I attempted to switch over to DBW when I did my swap and it was more headache than it was worth. To switch to DBC you would need a 99-02 PCM, wiring harness, DBC throttle body, pedal, and the bracket that bolts to the intake to hold the cable itself. Also you will need a new oil pressure sending unit with the older harness. acdelco part number: d1818a. A forum member, allblacksierra switched from dbc to dbw on a 4.3 to 5.3 swap if your'e interested in searching through some of his posts and continuing to do the conversion. My advice would be to stick with whichever drive by setup the truck came with from the factory. Hope this helps and good luck with the build.