I have replaced several distributors on these engines without using a Tech2 and never got any codes. It may take you a couple of attempts, but you can do it . . .
Here are specific instructions to install the distributor in an L31 Vortec engine:
1. Bring cylinder number one piston to Top Dead Center of compression stroke. (On the L31 there is a "v" cast right into the plastic timing cover right around the 2 o'clock position)
2. Remove Distrbutor cap
3. Make sure you have your distributor gasket in place
4. Align the indent hole on the drive gear (distibutor gear at bottom of shaft) with the paint mark on the distributor housing. (see below)
5. Align oil pump driveshaft with slot in distributor.
6. Align the flat side of the distributor (labeled "1" in the picture below) toward the front of engine.
7. Install the distributor and clamp but do not tighten clamp yet.
8. Once the distributor is fully seated on the intake manifold cover align the rotor contact with the number 8 pointer that is cast into distributor body. (see below) Make sure the center (middle) of the rotor contact is pointed directly at the peak of the triangle with the 8 stamped on it.
9. If the contact does not come within a couple degrees of the pointer then the gear mesh is off and the process needs to be repeated.
10. Tighten clamp once aligned (18 lb-ft)
11. Recheck the pointer alignment after tightening the distributor hold down clamp . . . The distributor can and will move during tightening of the clamp!! If it is off even a tiny bit you'll get the dreaded P1345 code.
12. Install cap (21 lb-in)