I'm no where near an expert on this, but from what I'm reading, you're having much the same issues I did on my 71 351C 4V, manual trans. My cam is a little different , but not that much. I believe you said you had a stock distributor. What size is the slot for the stop pin? I'm guessing it is an L15. If it is I think you'll find it is too big as it gives you 30* on the crank and why you only have 10* initial. If it is an L13, I think you can tune it with a small plastic sleeve on the pin and that will cut down the mechanical advance. Mine runs great on an L10 slot, 20* on the crank with 14* initial for a total of 34* mechanical. I also cut my vacuum advance down to 3-4* and I run it from the timed port on the carb. Another 'tweak' I might try is to add a sleeve to the pin and bump the initial up to 16*. Should still be about 34* total, but I'm hoping a bit better response, we'll see I guess. I recently posted a similar write-up the other day, perhaps you've already read it.
From what you posted above, my feeling is that the timing is not all in soon enough. I also am running 1 factory heavy spring and 1 Mr. Gasket 925d, but I want to play with tension to fine tune it. It gets to be fun when things start coming together, for me anyways.
Good luck, keeps us posted.
Geoff.
