89 C1500 Ignition Timing

Hoping somebody can help me out. I'm trying to set the timing on my 1989 C1500 Silverado and although it's now running terrific, I still don't think the timing is right. It has the 5.7 350 TBI. Here's what I did:
-Warmed truck up to normal operating temperature
- Checked initial timing with an inductive timing light and it was way off the chart... timing mark was at about the 11 o'clock position on the pulley (I figure about at least 25 degrees and it was running like it!).
- Turned truck off, disconnected the electronic spark advance wire under the junction box on the firewall
- Started truck and loosened the distributor hold down bolt
- Turned distributor until timing mark on pulley lined up with 0 degrees (according to the sticker under the hood) on the timing tab
- tighten the hold down bolt
- Re-checked timing and it was still at 0 degrees
- Turned truck off
- Re-connected the spark advance wire
- Disconnected the negative cable of the battery (for more than 30 seconds) to clear trouble codes
- Re-connected battery and started truck

Now, although it's running fine the timing mark on the pulley advanced all the way to about 14 degrees which is just past the 12 degree mark at the end of the timing tab. Should it have advanced that far??? I've done the above procedure 3 times all with the same result. Sorry to be long winded, I just want you experts out there to know exactly how I did it, which I'm pretty sure is by the book. Thanks for any help!

BTW... The awful spark knocking that I had is gone and it's running much better now, however it does feel a bit underpowered.

Spark Plugs are brand new (AC Delco) and gapped properly
ACCEL 8.8mm wires good
New Accel distributor cap and rotor
New MSD HEI Blaster coil
Edelbrock Performer TBI Intake (No Vacuum leaks)
New PCV and EGR Valves
Gibson Exhaust Headers
Low Temp Thermostat
New Gatorback drivebelt
Cold Air Intake
New IAC valve and coolant temp sensor

I've always run automatic-equipped small blocks advanced. Older 283s and 327s I set at 8 to 10 degrees BTD and I've ran my '88 K1500's 350 at 4 degrees BTD for quite a few years. I've seen some people run them 6 to 8 degrees with no issues (might begin to heat up a little depending on how you use your truck.) Advancing the spark with make a marked difference in the seat-of-the-pants-ometer and the OBD I computers won't screw up (not sure about OBD IIs.)

The EST will advance timing at warm idle to as much as 23º. That's completely normal. That is the reason to bypass the EST to set base timing. The ECM on these older systems has no crank trigger (CKP) so it is imperative that the base timing is set correctly. The ECM only advances timing a specified number of degrees over whatever the initial (base) timing might be. Timing advance at higher RPMs and at moderate load values can be as much as 48º, not counting the timing advance "adders" for power enrichment mode and offsets based on coolant and air intake temperature. If the programming for Highway Spark and Fuel Modes has been enabled, the timing advance can be well over 50º at part-throttle cruise.