Write down the timing at a few different rpms if you can. The early emissions distributors have very fast advance curves. If you're checking at 1000 rpm, it might already be 4-8* into the advance. I've not looked at your year so it may be like that or may be different. But lets say it is. Then if its at 5* BTC at 1000 rpm, it might be 0* at 800 rpm. So when put into gear, the engine slows a little and loses timing.
