As far as Cummins engines go, this is what I've been able to find out:
L-10 - 1982 - 1998
ISL - 1999 - Present
M11 - 1994 - 1999
M11 Plus - 1996 - 1999
ISM - 1999 - Present
N14 - 1992 - 2002
Signature 600 - Introduced in July 1997
ISX - ~1999 or 2000 introduction
If it were my T800 (which it's not), I'd go for the N14; but I think either the N14, 3406E, or Series60 would be appropriate.
Would a Caterpiller C15 be appropriate for this year of truck? When were the C12 & C15 introduced?