Poor idle and off-idle performance after distributor swap

Yes of course I have a timing light, but a small permanent marker mark on the back of the manifold where the v-can is pointing helps put the distributor back in within a few degrees of where it came out.

The comments about the V-can make sense. I didn't make the connection that the can fixes position of the advance plate. I assumed there was a stop that it rode against until vacuum advance was applied. I think the hole I drilled for the new v-can had to much slop, and was rattling back and forth at idle and causing the timing to vary.