I don't think you want to go smaller chamber size heads than 6x on a 455. That will create too much compression for pump gas. I think the timing cover is the same for 350-455 and I don't think it makes a difference. I'm not completely sure on that.
As for the 3core rad, I think Bronze Bird makes a good point with that. If you were driving at cruising speeds at no stop and go, MAYBE a clean, good flowing 3core rad would work. Since you mention you will be driving in much stop and go heavy traffic, I would suggest upgrading to a 4core aluminum rad.
With vacuum advance, it works with light load or cruising conditions. At full throttle at or above 3000 rpm, the vacuum advance reverts to mechanical advance and vacuum advance is lost. As we talked about before, that is something to look into.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."
