I've installed a HEI ignition in my 427. It's running, but not well. I can't seem to get it timed. The timing mark won't show up on the timing light, yes I have removed and capped the vacuum line.

I've tried advancing it until the engine misses and backing-off but, because I think it has a high-performance cam, it's hard to tell a miss from a 'rumble'.

Any ideas?  

I was one off on the distributor. That is why is wasn't showing up on the timing light.  

Pull the #1 spark plug, insert a piece of wire through the hole aimed at the piston, roll the engine by hand till the piston contacts the wire, carefuly continue to roll until you feel the piston stop at TDC. See if the line on the balancer is lined up to 0 on the scale. (Scale is normally off by a few deg.) If it's way off, either the elastomer has allowed the outer balancer ring to roll past it's original position, or the crank key is sheared. Mark a temporary line on the balancer and see if you can time the engine. If you can now time the engine, pull the balancer, and find out if key is broke or slipped ring.  JW.