Oh boy, I think you made a big boo-boo. I'll try to talk you thru it.
I have a MT2500 so I'm pretty familiar with it. You need a Domestic Cartridge that includes your year, and the 13a key. The 13 key does not function 100% correctly, it must be 13a.
There are two ways you can "see" fuel sync. One is on the all data function where you see the various voltages and inputs for all sensors as you scroll up and down with the wheel. That IS NOT the one you want. That one wanders all over to to timing advance from the pcm from various sensor input like the TPS, MAP, and O2 sensors.
The one you want is in the engine diagnostic section. I'm sorry I don't have my scanner in front of me to see the exact pathway. This test will instantly raise your idle rpm to 992 rpms (effectively 1000rpms) and hold it there to get rid of valve train fluctuation at idle. This is the test you want to use!
Be aware that if you get the disty twisted too far out of wack (+/- 15*), you will lose fuel sync and get a code for the cam or crank sensor. That's what likely happened.
Try to set the mounting ears of the disty cap (the screw mounts that the cap screws into on the actual distributor body, one is square and one is rounded) to about 3:15 and 9:15 o'clock to get you ballpark correct and then dial it in using the correct test. IIRC, square side goes on the left toward the center of the engine. Pay attention to the ears, otherwise you'll be 180* out of sync.
If you get it set correctly and the sync is still "walking" on you, you need to pull the disty and inspect the cam gear driveshaft, the oil pump driveshaft, and the mounting post of the disty to see if it got chewed up somehow. also look at the cam position sensor (also called the pick up sensor in the disty, round plate with the cord coming off of it sitting between the disty and the disty cap, yes it's removable!) Try replacing that if your sure everything is in order.
Good luck with it!