Motor Mystery

[pspeaks]

OK, I've got the flywheel thing and the harmonic balancer deal figured out and on order, but another mystery has presented itself.  My son has a 1998 Ford explorer with what looks like a 302 to me.  This Explorer has been in the family since new and I know for a fact it is the original motor; however, the number on the block above the starter is F1SE 8312, meaning a 1991 Thunderbird motor.  I can understand that late 91 could mean 92 but a 91 in a 98, makes me wonder if Ford ever had a clue what they were doing.  Still, a better choice than the 78 I have now; probably not a lot of differences during these years if any.  Is it possible it's still a roller motor?  It's still at my son's house and I plan on picking it up next week or so.

[pspeaks]

I should have added that NO, I'm not using the automatic transmission.