OK, I ran the block # 8933002665 and it does come back as a 87-90 4.0 That's good news. I was basing my thought that it may be a 258 due to the only picture I have been able to find in a google search of a I-6 block with 3 big (2") and 2 small (about 1") freeze plugs was stated to be a 4.2-258. My original motor has 5, 2" freeze plugs on the manifold side OOOOO but the other motor looks like this OoOoO. Good news is the block casting # comes back to a 4.0. So I guess the 4.0 came in both freeze plug layouts although all the pictures I have looked at, tons of them, all show 5 large plugs. The motor in question is an injected serpentine set up. From what I have found on this site the 4.2 is a carbureted engine but can be converted.
Thanks for the help.