You say you have tried timing changes, have you removed vacuum can from distributor and regraphed it to suit the motor? Without the correct advance curve you could be doing serious damage to your motor at those rpm.

What about fuel flow and pressure, do you have a pressure gauge hooked up at top revs? You want about 2-3psi at idle never dropping below 1.5psi at high revs. If it does drop you need more flow ie. larger pump or 2 inline.

Hope this helps.