Valve Clatter - Timing ?
 

I hear some valve clatter when it hit 2nd and third gear on a slight incline on my 305 K5 Blazer. I've bumped the timing from 4 to 8 to 12 without major improvement although 12 does sound better than 8 degrees. Yesterday I filled up with 89 octane to give it a try. Again some improvement. This is a newly rebuilt motor with about 2500 miles on it.

Ideas.

Just to rule out the obvious, you have adjusted the valves correct?

To be honest, the valve adjustment crossed my mind but I haven't done it yet. I really thought it was the timing. Thought I'd hear valve clatter all the time if they needed adjusted.

I can't remember adjusting the valves after installing the 350 in the 70 C-10.

Also, this was a motor swap not a motor I rebuilt.

Is it valve clatter or spark knock you are hearing? By your description, sounds like spark knock to me. Try lowering your timing to 6 BTDC and check it. 305's are the worst engines ever built by GM for this. The 305 combustion chamber was the cause. This is the very reason GM used the ESC distributors with knock sensors in LE9 trucks.