Basically a working VA will retard the timing with decrease of manifold pressure.
You can check your VA with a Vacuum gauge and suck on it to determine the starting and max. range in Hgs.
Distributors and VAs look similar or alike for these engines over the years.
However there are quiet some differences.
Check you engine year's manual to find the correct part numbers.
There are also different numbers for cars and trucks.
Base timing also plays a role to get it all tuned properly.
If your engine has been upgraded from stock, you may have to compensate or modify the distributors springs and weights and/or get other VAs with a different range.
Another thing to think about is if you are using a PCV system with a valve that drops too much Hg.
If you go by todays PCV valve suggestions for this engine, that's the case.
After experimenting I found the perfect valve with the least drop. You can search the archive for a post I did last year, which has the part number I ended up with.
Of course always check for vacuum leaks.
