I pulled the distributor on the 455, and with a mirror in the distributor hole, I saw a about 3/4 inch or so plug installed. Is that the one your referring to?

Also, I have a 350 on my engine stand with the timing cover off and could see 2 small 1/2 inch plugs behind the timing chain. Are those the front ones your referring to?

If so, then it sounds like I'll pull the timing cover off the 455 and see if the 1/2 inch plugs are istalled.