Hi,
Have you checked the compression, because it's unlikely to be 10:1 with the 906 heads, more likely to be somewhere around 8.5:1, especially if the pistons are a lot below deck.
If you calculate the compression don't trust the advertised cc's, the open chamber 906 are over 90cc's uncut.
With just 8.5:1 compression the MP 284/484 is a lot of cam.
You could run the dizzy on manifold vacuum if you haven't done so already.
This will give more advance at idle helping the engine making more vacuum.
You could also advance the cam a couple of degrees.
This is more work however, cause you need to take the timing cover off.
Was the camshaft degreed in when you installed it, or did you just line up the dots?
Or install a "smaller" camshaft with less duration and earlier intake valve closing.
Cheers,
Frederick