Is it running or are you trying to do an initial timing? The timing specs really aren't very relevant. Gasoline had a lot lower octane in the 1950's and with today's octane the engine will be a lot happier with a little more advance. The 503 spanned the entire 1950's and the newer ones had more hp than the older ones because octane rose during the 1950's. If it is running advance the timing a little until it starts pinging and then retard it a little. If you are trying to get an initial timing, remove the no. 1 plug and rotate the engine until gets to a compression stroke on the no. 1. Then use a flash light to see when the piston is all the way up. That will be pretty close to TDC and it should start. You can keep advancing it until it pings and then retard it a little. That's how I did it over 60 years ago and it will still work. I was a 14 year old kid when I built my first GMC engine in 1957.