I've never even heard of a 7.3 IH in a B600...it was pickup-only for Fords. A B600 could also have:
5.9 B-series Cummins (I-6).
8.2 Detroit (V
3208 Cat (V8, very unlikely)
8.3 Cummins (I-6, very, VERY unliklely)
6V53 or 6V71 Detroit (V6, very, very unlikely...but would be pretty cool)