What about the balance of the motor? Lets say you find a balancer and timing cover that works, you need to make sure it was balanced with the rest of the rotating assembly.

For example I had a 306 in a black hatch back in 05. When I built the motor I had given my machine shop every part i had including an SFI flexplate, months later the flex plate broke and had to replace it with another of the same part number and it had a horrible vibration from 2500-3500. I pulled it back out and had the two of them spun and the one was an ounce and a half off.

Unless your still in mock up stage then nevermind... lol